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PMB rebuild and bleeding
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Cptn. Calzone
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PostPosted: Sat Mar 04, 2017 6:21 pm    Post subject: PMB rebuild and bleeding Reply with quote

Recently had PMB rebuild my calipers on my 67 912.They came back looking beautiful, loaded with new pads springs and clipshardware.I filled each with ATEBlue and mounted them on my car.Now I am bleeding...and bleeding...and bleeding and cannot sem to get a CONSISTENT pedal!? The first push heads down further in travel and then pumps up , but still not quite what I like.I am heading back tommorrow for more.I did get all the old trash and air/foam out of the system.Now the fluid is clear and free of any funk, yet the good pedal is elusive.Any and all help; greatly appreciated.
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raygreenwood
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PostPosted: Sat Mar 04, 2017 7:13 pm    Post subject: Re: PMB rebuild and bleeding Reply with quote

Cptn. Calzone wrote:
Recently had PMB rebuild my calipers on my 67 912.They came back looking beautiful, loaded with new pads springs and clipshardware.I filled each with ATEBlue and mounted them on my car.Now I am bleeding...and bleeding...and bleeding and cannot sem to get a CONSISTENT pedal!? The first push heads down further in travel and then pumps up , but still not quite what I like.I am heading back tommorrow for more.I did get all the old trash and air/foam out of the system.Now the fluid is clear and free of any funk, yet the good pedal is elusive.Any and all help; greatly appreciated.


Hang loose and I will get into this deep with you in the am. This is usually a master cylinder issue with air bubbles. Its not a caliper problem......but the fact that the whole system was opened and largely drained is where you problem is. Lots of cars brake systems have this difficulty.
I will lay out what you need ro do in the am. Ray
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Cptn. Calzone
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PostPosted: Sat Mar 04, 2017 7:38 pm    Post subject: Re: PMB rebuild and bleeding Reply with quote

Thank yopu Ray.I am begining to question the master which is relatively new and a 911 one at that.I am getting more fluid and heading back to the man cave tommorrow so any input is greatly appreciated.
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raygreenwood
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PostPosted: Sun Mar 05, 2017 8:52 am    Post subject: Re: PMB rebuild and bleeding Reply with quote

Ok.....sorry...had to get some coffee.

So.....you had calipers rebuilt at PMB. They do good work. In general the calipers and/or wheel cylinders will not be the cause of not being able to get pedal during bleeding.....but there are a couple of caliper related items ai will mention.

1. Make sure the front calipers......if they only have one bleeder valve.....that the bleeder valve is at the top of the caliper. It can make getting the air out of one circuits more difficult.

2. If you have not installed Speed Bleeder valves.....you are missing out. Worth every penny. They can solve part of this problem.

The stock bleeder valves can be a problem if you are using a self bleeder hose with the check valve in the end.....or using a vacuum bleeder setup like mighty vac (which I do not recommend). They pull air in through the threads. I will tell you what this causes in a moment.
You can fix this by doing two things:
A. if you do not have ready access to Speed Bleeder valves which have a brake fluid resistant thread sealant.....you can use a single wrap of teflon tape. BUT.....do not let it go any farther down than the thread.

B. When bleeding...ONLY crack the bleeder valve open 1/4 to 1/5 of a turn. You want it to be restricted so it requires you to build high pressure on the pedal to force the fluid past the barely open valve.

So what is really happening here?

So you had to nearly drain the system to replace the calipers. What is happening is that the at-rest fluid friction....literally the viscosity of the fluid a and its stickiness..... in the lines between master cylinder and calipers.....is as high or higher than the fluid friction between master cylinder and fluid reservoir.

This means that when you push the pedal.....it does not build much pressure yet because of air in the system...because the air compresses. So you do not really move fluid and air through the system because it does not properly inflate the cups in the master cylinder.....so the air bypasses the MC piston on the pressure stroke....going into the compensation port area of the piston between the two cups. So you build no pressure and vent no air.

This is why you need to control the orifice size on the bleeder valve so you build high pressure to make sure the cups inflat in the MC and you are not just moving air back and forth in the MC.

Likewise....when you release the pedal.....on its return stroke.....what fluid there is in the master cylinder circuit.....does not build enough pressure to open the brass flap valves.....partly because there is already air between the pistons in the MC...and partly because you may be leaking air back into the circuit at the bleeder screw threads....which acts as a vacuum breaker.....and lets the fluid draw back up the line between caliper and MC.......so the master cylinder does not draw in fluid from the reservoir.

The way to get around this......is to as mentioned .....make sure that your bleeder valves are set to create high pressure on the pressure stroke of the pedal.

When you are just starting this process....because of the bleeder valve setting.....you should get a hard slow pedal stroke.
If you dont have speed bleeder valves...which are ball check valves......when it reaches the bottom of the stroke.....use a rod or a wood dowel to hold the pedal on the floor. Get out and close the bleeder valve.

Now.....when you bring the pedal up....bring it up very slowly.....take about 10 seconds to let it up. This keeps the very tenative replenishment flow of fluid from the very low suction.....from breaking and cavitating.
In short...in these early stages of bleeding when the fluid friction is nearly equal on both sides of the piston.....moving the piston too quickly causes the fluid stream to cease because it cavitates and pulls in air that is already close by in the piston.

One other item......but this should not be your problem if you have already had the MC installed for a while......but when installing a new MC....make sure that the plastic elbow that pops into the rubber grommet at the top.....is not forced down so far that the mouth of the plastic elbow is flush with the bottom of the bore under the grommet. This blocks fluid flow just like have your soda straw square on the bottom of your drinking glass....and not being able to suck anything through it.

After about 3-5 cycles of hard pressure down....and slow even upstroke pressure.....the MC will start drawing better and you should be able to bleed normally.

If you have a pressure bleeder system.....it greatly helps alleviate this condition. Ray
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Cptn. Calzone
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PostPosted: Sun Mar 05, 2017 9:58 am    Post subject: Re: PMB rebuild and bleeding Reply with quote

Thank you Ray
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BK911
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PostPosted: Wed Mar 22, 2017 12:49 pm    Post subject: Re: PMB rebuild and bleeding Reply with quote

With rebuilt calipers, most of the time it takes a few rides to get the brake pistons in the proper place. They usually start out too far from the rotors. Every time you apply the brakes, the pistons will rest a little closer to the rotor, giving a harder pedal.
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raygreenwood
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PostPosted: Wed Mar 22, 2017 2:02 pm    Post subject: Re: PMB rebuild and bleeding Reply with quote

BK911 wrote:
With rebuilt calipers, most of the time it takes a few rides to get the brake pistons in the proper place. They usually start out too far from the rotors. Every time you apply the brakes, the pistons will rest a little closer to the rotor, giving a harder pedal.


To clarify.....you mean the piston retraction function of the calipers can take a number of cycles before getting the exact pad to rotor gap.....correct?

This is true. Until the onner ring seal gets slightly broken in to the piston.....your pad gap to rotor may be a little odd for a few miles.

This is actually mainly to do with the pad itself.....as there is some compression and temperature expansion and outgassing of the pad until it gets bedded to the rotor (worn down until its fully flush with the rotor surface).
The inner ring seal will become a little more elastic as gets heated a few times.....but largely returns or retracts the same amount.
However until the pad beds in....it wears down significantly on each drive as it wears the peaks off the pad surface ....and that level of wear is frequently in between the standard retraction level of the inner ring seal during this period.

Yes....during the brake in period it can make the pedal feel soft...as that few thousandths of movement is multiplied by the length of the brake pedal.

However.....this will have effect on bleeding. Ray
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chrisflstf
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PostPosted: Wed Mar 22, 2017 3:42 pm    Post subject: Re: PMB rebuild and bleeding Reply with quote

Anybody know what Speed Bleeder valves work on VW's?
Russells doesnt show many for older drum brake systems - Type II, I'm interested in

http://www.russellperformance.com/mc/speed-bleeders/
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raygreenwood
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PostPosted: Wed Mar 22, 2017 8:41 pm    Post subject: Re: PMB rebuild and bleeding Reply with quote

chrisflstf wrote:
Anybody know what Speed Bleeder valves work on VW's?
Russells doesnt show many for older drum brake systems - Type II, I'm interested in

http://www.russellperformance.com/mc/speed-bleeders/


Which VW's?.

The late vw type 3.....411 and 914 to August of 72 and 412 ane 91r after August of 72 ....use the same speed bleeder for front calipers.

That part # is in here if you scroll down

https://www.thesamba.com/vw/forum/viewtopic.php?t=658176

I believe the factory # for the long bleeder valve was 311 615 273. The speed bleeder part # IIRC is SB7100.

NOTE: there are a couple of speed bleeder valves that fit the front calipers....but this one is the only one that is the correct length. Ray
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chrisflstf
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PostPosted: Thu Mar 23, 2017 5:36 pm    Post subject: Re: PMB rebuild and bleeding Reply with quote

Quote:
Which VW's?.


This would be for a Type II, with stock 67 brakes - F/R

BTW, your write up on caliper rebuild was top notch. I read it all, even though I dont need to rebuild any. Some really great ideas. Thx for your input
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raygreenwood
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PostPosted: Thu Mar 23, 2017 7:56 pm    Post subject: Re: PMB rebuild and bleeding Reply with quote

chrisflstf wrote:
Quote:
Which VW's?.


This would be for a Type II, with stock 67 brakes - F/R

BTW, your write up on caliper rebuild was top notch. I read it all, even though I dont need to rebuild any. Some really great ideas. Thx for your input


Thank you! I will be rivising a few items soon.....most notably.....directions to NOT acid wash the pistons.

They are as I have discovered....nickel plated. I will be replating mine. You can get away with what I did to mine but it,absolutely means a tear down every 2 years to inspect for piston rust on the outside of the outer seal. Not a big deal for some.....but a pain.

I will find your part # by morning. Ray
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