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Brake bleeding - what am I missing?
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veedweeb
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PostPosted: Mon May 24, 2021 1:39 am    Post subject: Brake bleeding - what am I missing? Reply with quote

I've just changed the front calipers, and the front and rear flexible hoses so I need to do a full brake bleed.

I've used one of the brake bleeders that pressurises the system using the spare tyre, and also a vacuum bleeder that uses compressed air to suck fluid through the pipes.

Having run about 2.5litres of fluid through, there are no air bubbles coming out of the bleed screws but the brake pedal goes right to the floor and doesn't pump up either.

I'm going to try a 2 person approach next to bleed them the traditional way, but am I missing something? It's a '72 car with dual circuit brakes and stock discs/drums
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PostPosted: Mon May 24, 2021 4:08 am    Post subject: Re: Brake bleeding - what am I missing? Reply with quote

Are your rear brakes adjusted correctly? How does your parking brake feel? If you pull your park brake until it's tight does the pedal feel differently?

Also, did you put the correct side front calipers on? Bleed screws are at the top?
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PostPosted: Mon May 24, 2021 4:43 am    Post subject: Re: Brake bleeding - what am I missing? Reply with quote

old_man wrote:
Are your rear brakes adjusted correctly? How does your parking brake feel? If you pull your park brake until it's tight does the pedal feel differently?

Also, did you put the correct side front calipers on? Bleed screws are at the top?


I haven't tried the parking brake yet so I don't know about that one.

The calipers are the later type with 2 bleed screws each so they're the same on both sides
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PostPosted: Mon May 24, 2021 6:47 am    Post subject: Re: Brake bleeding - what am I missing? Reply with quote

Tell us about the master cylinder.

How old
What brand

Also.....check pedal freeplay adjustment.
Ray
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PostPosted: Mon May 24, 2021 7:11 am    Post subject: Re: Brake bleeding - what am I missing? Reply with quote

raygreenwood wrote:
Tell us about the master cylinder.

How old
What brand

Also.....check pedal freeplay adjustment.
Ray


The master cylinder itself is unknown but it did work before I swapped the calipers.

I bought the car in July last year, and the brakes weren't great but they worked. I tried to bleed them but I managed to snap a couple of bleed screws in the front calipers. At that time I noticed the calipers were mismatched so I sourced another pair and had them rebuilt/refurbished rather than try and fight to get the broken bleed screws out only to still have mismatched calipers.
So this weekend I swapped to the rebuilt calipers and changed the flexi lines at the same time. most of the fluid drained out while I was doing this.

Now there's no resistance at the pedal at all - not free play, but no hydraulic pressure, if that makes any sense.
Using my pressure bleeder, nice clean fluid comes out of all the bleed screws with no sign of any air in it at all.

In all my years of working on cars, I've never come across this before and I wondered if there's a specific quirk on Type 3s that I'm missing?
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PostPosted: Mon May 24, 2021 10:27 am    Post subject: Re: Brake bleeding - what am I missing? Reply with quote

Bentley Type 3 manual says to bleed the fronts first, then the rears in Fig. 9.3 Chapter 2.
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PostPosted: Mon May 24, 2021 11:54 am    Post subject: Re: Brake bleeding - what am I missing? Reply with quote

Mike Fisher wrote:
Bentley Type 3 manual says to bleed the fronts first, then the rears in Fig. 9.3 Chapter 2.


Yup. Adjust the adjusters tight (not super tight, but just barely tight, before bleeding. Then bleed the RF, and LF, then the RR followed by the Left Rear. Then bleed the rears, followed by backing off the adjusters enough so you can spin the tire. What you're looking for is not letting the wheel cylinders to move (or barely move) while getting the air out while bleeding.

What you're looking for is 3 clicks on the hand brake to set them (parked if the cables are adjusted correctly), and a firm pedal when you step on the brakes.
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PostPosted: Tue May 25, 2021 11:25 am    Post subject: Re: Brake bleeding - what am I missing? Reply with quote

veedweeb wrote:
raygreenwood wrote:
Tell us about the master cylinder.

How old
What brand

Also.....check pedal freeplay adjustment.
Ray


The master cylinder itself is unknown but it did work before I swapped the calipers.

I bought the car in July last year, and the brakes weren't great but they worked. I tried to bleed them but I managed to snap a couple of bleed screws in the front calipers. At that time I noticed the calipers were mismatched so I sourced another pair and had them rebuilt/refurbished rather than try and fight to get the broken bleed screws out only to still have mismatched calipers.
So this weekend I swapped to the rebuilt calipers and changed the flexi lines at the same time. most of the fluid drained out while I was doing this.

Now there's no resistance at the pedal at all - not free play, but no hydraulic pressure, if that makes any sense.
Using my pressure bleeder, nice clean fluid comes out of all the bleed screws with no sign of any air in it at all.

In all my years of working on cars, I've never come across this before and I wondered if there's a specific quirk on Type 3s that I'm missing?


All the advice given is spot on.

Some other items that can come up...with master cylinders that are overall good but have a little age on them....as well as some MC's in cars where the (from what I can tell)....wheel cylinders or calipers are high volume/long stroke.

With age....the EPDM rubber pressure cups become slightly stiffer. This means that as they age...due to shape....and confinement in the bore (especially if the car has not been driven for a while)...the natural tendency of these cups is to flex inward away from the cylinder bore walls.

This means that....until they can build up "some" pressure.... they don't seal well...and so they do not readily build up pressure Wink ...catch 22.

An advanced and extreme version of this is what you get with very OLD NOS cylinders...that will be hard to bleed and once they are operating....have the "pedal drops to floor" syndrome.

But in this case...moderate miles and not NOS....but just slightly hard to seal.....if you cannot easily build pressure....the seal cups do not flatten the brass flap valves tightly against the compensation port fluid return holes.

So....as you pump...the fluid just passes back and forth internally inside of the MC....and cannot move the air. To add to this....it also cannot readily draw new fluid in.....so....the pressure bleeding is helpful.

If you do not have a pressure bleeder....what REALLY helps...is to sit on the ground on the drivers side....and actuate the pedal with your hand. Start out with rapid, hard jabs to the floor. (to inflate and seal the cups)...but let the pedal up VERY slowly...really like 10-12 seconds.

This helps...because with air in the system and much more fluid surface tension on the outlet side....letting the pedal up quickly cannot draw fluid in because there is more volume and surface tension on the downstream side....so the master cylinder literally draws fluid BACK from the calipers and wheel cylinders instead of the reservoir. Letting up very slowly helps to draw fluid in from the reservoir.

Part of what makes this happen is the other issue..... that as Bobnotch noted.....if the wheel cylinders are allowed to move at all....it acts as a pressure relief....so fluid pressure to seal the cups and ports...cannot readily be brought to bear....and it allows fluid to contract/return back to the MC from the wheel cylinders just like I am noting above.

Once you get it drawing or start using a pressure bleeder.....about 10 fast, hard pumps with all wheel cylinder bleeder valves closed. You may not feel it....but it builds up some pressure. Then....hold this pressure by putting a rod against the pedal.

Then get out and open the bleeder valve for a second or two. Close it. Get back in....and let the pedal up very slowly to draw in fresh fluid from the reservoir and not the wheel cylinders.

Repeat this process. This is the absolute best method I have found for used systems like yours that match your description. Ray
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veedweeb
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PostPosted: Thu May 27, 2021 2:36 am    Post subject: Re: Brake bleeding - what am I missing? Reply with quote

Thanks for all the replies, everyone. Some good advice there, and some techniques I haven't tried yet.

I'm having a short break this weekend so it'll probably be another week before I can get to my workshop to try again but I'll report back then.

I didn't mention that this is a RHD car (I'm in the UK) and new RHD master cylinders are not available. LHD ones are but that would mean modifying the hard lines - not a disaster but I'd rather not do that if I don't have to.

Thanks again Smile
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PostPosted: Thu May 27, 2021 4:34 am    Post subject: Re: Brake bleeding - what am I missing? Reply with quote

When I was having bleeding brake trouble the piston was sticking at the bottom of its travel in the master cylinder. It was the wrong diameter MC too so changed it and they bled easily.
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PostPosted: Tue Jun 15, 2021 5:24 am    Post subject: Re: Brake bleeding - what am I missing? Reply with quote

So, after trying and trying to get a decent pedal feel, I have finally given up and bought a new master cylinder.

As I mentioned previously, new RHD ones are not available so I had to order a LHD one.

This is what has just turned up. It looks exactly the same as the one I'm taking off which is good because that means I don't have to alter the hard lines.

But the mounting flange is threaded, and the bore (at least where the clip that holds the piston in) is about 24mm. The existing one was also threaded on the flange but I can't recall what the bore measured.

Is this the wrong part? It came in a box with 311-611-015K as the part number.

Image may have been reduced in size. Click image to view fullscreen.


Image may have been reduced in size. Click image to view fullscreen.


Image may have been reduced in size. Click image to view fullscreen.

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PostPosted: Fri Jun 18, 2021 7:37 am    Post subject: Re: Brake bleeding - what am I missing? Reply with quote

You will need to drill out the threaded holes 8mm clearance.
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PostPosted: Fri Jun 18, 2021 8:14 am    Post subject: Re: Brake bleeding - what am I missing? Reply with quote

veedweeb wrote:
So, after trying and trying to get a decent pedal feel, I have finally given up and bought a new master cylinder.

As I mentioned previously, new RHD ones are not available so I had to order a LHD one.

This is what has just turned up. It looks exactly the same as the one I'm taking off which is good because that means I don't have to alter the hard lines.

But the mounting flange is threaded, and the bore (at least where the clip that holds the piston in) is about 24mm. The existing one was also threaded on the flange but I can't recall what the bore measured.

Is this the wrong part? It came in a box with 311-611-015K as the part number.

Image may have been reduced in size. Click image to view fullscreen.


Image may have been reduced in size. Click image to view fullscreen.


Image may have been reduced in size. Click image to view fullscreen.


That's because it's a disc brake bug master cylinder. The threaded holes are the give away. Also England and other parts of Europe got disc brakes on their beetles while the USA only got drums thru the end of production.
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PostPosted: Mon Jun 21, 2021 1:29 am    Post subject: Re: Brake bleeding - what am I missing? Reply with quote

This thing still isn't sorted!

I queried the m/c with the supplier and they say it will definitely work for my car (they're a really reputable VW parts supplier here and I've been using them for years).

The m/c itself is definitely 19mm bore - I took the piston out and measured the bore myself.

I have done a bench bleed on it. By that I mean I fitted it to the car and made adaptors to run the outlets via silicon hoses up the reservoir and pumped until the fluid looked clear of air bubbles.

Next I fitted the actual hard lines to the outlets and bled at all the wheels using an assistant to pump the pedal while I opened and closed the bleed screws. No air was visible.

That didn't work.

So next I connected my pressure bleeder to it. It's the simple kind that pressurises the reservoir with fluid. I've used it many times before and always had good results.
While it was pressurised, we repeated the pedal up, pedal down, screw opened, screw closed procedure and still no real pedal feel.

The brakes do operate, barely. Turning the wheels by hand, then applying the brakes stops the wheels so all the pistons are moving but with the car on the ground they barely stop the car from 10mph.

Pumping the pedal does firm it up noticeably which suggests to me that there's still air in it somewhere.

I think my next step is to use the pressure bleeder again, but this time instead of the pedal up, pedal down, screw opened, screw closed procedure I'll get my assistant to pump the pedal up and down while each screw is opened.

Other than that, I'm all out of ideas. Sad
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PostPosted: Mon Jun 21, 2021 9:04 am    Post subject: Re: Brake bleeding - what am I missing? Reply with quote

Ray's advice has always worked for difficult bleeding jobs, even with the pressure bleeder, I've had to do this to get the stubborn air bubbles moving out

Quote:
If you do not have a pressure bleeder....what REALLY helps...is to sit on the ground on the drivers side....and actuate the pedal with your hand. Start out with rapid, hard jabs to the floor. (to inflate and seal the cups)...but let the pedal up VERY slowly...really like 10-12 seconds.

This helps...because with air in the system and much more fluid surface tension on the outlet side....letting the pedal up quickly cannot draw fluid in because there is more volume and surface tension on the downstream side....so the master cylinder literally draws fluid BACK from the calipers and wheel cylinders instead of the reservoir. Letting up very slowly helps to draw fluid in from the reservoir.

Part of what makes this happen is the other issue..... that as Bobnotch noted.....if the wheel cylinders are allowed to move at all....it acts as a pressure relief....so fluid pressure to seal the cups and ports...cannot readily be brought to bear....and it allows fluid to contract/return back to the MC from the wheel cylinders just like I am noting above.

Once you get it drawing or start using a pressure bleeder.....about 10 fast, hard pumps with all wheel cylinder bleeder valves closed. You may not feel it....but it builds up some pressure. Then....hold this pressure by putting a rod against the pedal.

Then get out and open the bleeder valve for a second or two. Close it. Get back in....and let the pedal up very slowly to draw in fresh fluid from the reservoir and not the wheel cylinders.

Repeat this process. This is the absolute best method I have found for used systems like yours that match your description.

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PostPosted: Fri Jun 25, 2021 2:22 am    Post subject: Re: Brake bleeding - what am I missing? Reply with quote

W1K1 wrote:
Ray's advice has always worked for difficult bleeding jobs, even with the pressure bleeder, I've had to do this to get the stubborn air bubbles moving out

Quote:
If you do not have a pressure bleeder....what REALLY helps...is to sit on the ground on the drivers side....and actuate the pedal with your hand. Start out with rapid, hard jabs to the floor. (to inflate and seal the cups)...but let the pedal up VERY slowly...really like 10-12 seconds.

This helps...because with air in the system and much more fluid surface tension on the outlet side....letting the pedal up quickly cannot draw fluid in because there is more volume and surface tension on the downstream side....so the master cylinder literally draws fluid BACK from the calipers and wheel cylinders instead of the reservoir. Letting up very slowly helps to draw fluid in from the reservoir.

Part of what makes this happen is the other issue..... that as Bobnotch noted.....if the wheel cylinders are allowed to move at all....it acts as a pressure relief....so fluid pressure to seal the cups and ports...cannot readily be brought to bear....and it allows fluid to contract/return back to the MC from the wheel cylinders just like I am noting above.

Once you get it drawing or start using a pressure bleeder.....about 10 fast, hard pumps with all wheel cylinder bleeder valves closed. You may not feel it....but it builds up some pressure. Then....hold this pressure by putting a rod against the pedal.

Then get out and open the bleeder valve for a second or two. Close it. Get back in....and let the pedal up very slowly to draw in fresh fluid from the reservoir and not the wheel cylinders.

Repeat this process. This is the absolute best method I have found for used systems like yours that match your description.


When I was down at the workshop the other day, I was trying to remember Ray's advice. I'd forgotten to take my phone with me so I couldn't look it up.
I'll definitely try it next time I'm there.

In the meantime though, I think I might have realised what the problem (or at least part of it) is.

These are my refurbished calipers:

Image may have been reduced in size. Click image to view fullscreen.


When fitted to the car, the lower bleed screws are completely below their piston. It'll be impossible to get the air out of that side of the caliper so I'm going to unbolt the calipers from the spindles and rotate them while I bleed them to make sure the bleed screw is the highest point.
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PostPosted: Fri Jun 25, 2021 7:09 am    Post subject: Re: Brake bleeding - what am I missing? Reply with quote

Your calipers appear to have TWO bleeder valves on each one right?

The LOWER bleeder screw is not used at all. Only the top one. Also bleed the front first. This may go contrary to stated habit.....but you are using a beetle MC....right? That can change the dynamic.....because of the spring pressures on the piston set for each circuit. Depending on how the brake bias is set up......it can be just the outer piston moving for a few millimeters before the inner one moves at all....or they can both move somewhat at the same time.

Early in bleeding before pressure gets built up it can allow internal bypass between pistons circuits.....meaning that instead of bleeding you are just moving fluid back and forth.

Check your pedal free play carefully. Make sure that when the pedal is all the way up....there is physically a gap between the end of pushrod and the outer piston.

Also.....go to my master cylinder rebuild document in the 411/412 forum . Scroll down to the final assembly. Note the modification to the plastic elbows on the inlet tubes from the reservoir.
Its been a common problem for eons that the bottoms of these elbows when they pass through the grommet in the MC....csn end up dead flat against the cssting and block fluid draw
I file little notches in the very bottom edge of these plastic elbows.

https://www.thesamba.com/vw/forum/viewtopic.php?t=679160

On systems where there is some difficulty in bleeding.....I start with the front first because the line have shorter distance. This allows some small success FIRST....so at least that circuit starts pressurizing and inflating the cups a little ti bring piston efficiency up.

Ok....also on virtually ALL calipers and wheel cylinders.....to varying degrees......I find that the bleeder valve threads....LEAK AIR....when the valve is open. This is especially true with very old or remanufactured wheel cylinders and calipers.

This is because over the decades they got rusty....lost metal....and are a loose fit with age and all the reman process does is wife wheel them and clean them up. So.....if during bleeding.....you push the pedal down.....and do not hold it down while you close the bleeder valve.....it will ALWAYS draw some air back in through the bleeder valve threads.

This air bubble....allows surface tension to simply pull the fluid back to the master cylinder when you pull the pedal back up.....so the fluid and air in the lines just moves back and forth.....for hours.

So.....TWO THINGS:

1. Wrap all if the bleeder valves with a single layer of Teflon tape.....ON THE THREADS ONLY! Do not get the tape near the smooth lower end is with the sealing point.

2. DO NOT EVER....open the bleeder valve more than about 1/8th of a turn. You want the very smallest opening you can possibly produce.....so that any stroke you make with the pedal....builds up SOME pressure. This way it always inflates the MC piston cups.....which makes them seal better in the bore....have less internal bypass and moves more fluid.

And.....when you build even small amounts of pressure....this squeezes and compresses the air bubbles in the line.....making them smaller....separating them from the brake line wall and making them easier to move in the fluid stream.

You MUST....have a board or rod.....to hold the pedal to the floor while you get out and close the bleeder screw.....BEFORE you get back in and let the pedal up.

Once you have done the hand push and slow lift of pedal for a little while......do about 20 to 30 full strokes....one at a time....like this:

Bleeder valve closed

Pump pedal up rapidly maybe 10 times...even if you feel no pressure

Hold to floor hard and put in rod to hold it there.

Get out and crack bleeder open for a second and close it tight

Get back in and let pedal up SLOWLY.....8-10 seconds.

Repeat.

Ray


Last edited by raygreenwood on Fri Jun 25, 2021 9:12 am; edited 1 time in total
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PostPosted: Fri Jun 25, 2021 8:49 am    Post subject: Re: Brake bleeding - what am I missing? Reply with quote

what ray said

I'll add then when you are bleeding, tap the calipers with a soft mallet. All over. you'll get the tiny bubbles out quicker that way
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PostPosted: Fri Jun 25, 2021 9:06 am    Post subject: Re: Brake bleeding - what am I missing? Reply with quote

Erik G wrote:
what ray said

I'll add then when you are bleeding, tap the calipers with a soft mallet. All over. you'll get the tiny bubbles out quicker that way



ooooh....good technique!

I cannot remember as much about the type 3 hard line routing...because its been so lone since I owned one. But in type 4 cars....the lines are very "circuitous". There are higher points in the hard lines and a few hard bends.

Small bubbles get stuck here due to surface tension...and fluid bypasses the bubbles. So though you get good clean fluid out of the beeder valves...there an still be small bubbles in the system that compress and relive pressure.

So...though get get pretty good brakes...you get a peddle that feels...."less than" it should with all new components. I found that rapping on the hard lines where you can reach them and a few good hard drives...anda re-bleed....has to happen. Ray
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PostPosted: Sat Jun 26, 2021 6:34 am    Post subject: Re: Brake bleeding - what am I missing? Reply with quote

veedweeb wrote:


In the meantime though, I think I might have realised what the problem (or at least part of it) is.

These are my refurbished calipers:

Image may have been reduced in size. Click image to view fullscreen.


When fitted to the car, the lower bleed screws are completely below their piston. It'll be impossible to get the air out of that side of the caliper so I'm going to unbolt the calipers from the spindles and rotate them while I bleed them to make sure the bleed screw is the highest point.


The bottom or lower bleed screws are for draining brake fluid out of the caliper. Having 2 bleeder screws also allows companies (rebuilders) to only have to stock 1 caliper for both sides.
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