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Bleeding brakes - nothing but bubbles in the rear
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Syncroweekender
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PostPosted: Sun Apr 24, 2016 9:51 pm    Post subject: Bleeding brakes - nothing but bubbles in the rear Reply with quote

I replaced the master brake and clutch cylinders and now I'm having a devil of a time getting the rear brakes bled. I tried to bench bleed the brake master but one of the plastic fittings in the kit I got didn't seal well so the aft most fitting bubbled on every stroke. I put the master cylinder in and started to bleed the system with a Motive pressure bleeder, starting pressure around 8 - 10 psi. The rear brakes just keep bubbling. I've put over a quart through each of the rear brakes and still have a steady stream of bubbles. I bled the fronts because I got tired of watching the all the bubbles in the rear and they both bled fine.

The brake and clutch pedals feel pretty firm but I haven't started the van yet, as the dashboard is still out of it. I did a quick look at the master cylinders and brake lines from front to back and I didn't see any evidence of leaking. Anyone have any suggestions?

My van is an 86 syncro with Smallcar front and rear discs and I just put in a BMW brake booster with the new master cylinder. The rest of the system is stock.

Thanks.
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PostPosted: Mon Apr 25, 2016 4:48 am    Post subject: Re: Bleeding brakes - nothing but bubbles in the rear Reply with quote

Step away from the power bleeder and do it the old fashioned manual way.

I bet you will see success.

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PostPosted: Mon Apr 25, 2016 7:00 am    Post subject: Re: Bleeding brakes - nothing but bubbles in the rear Reply with quote

Open one bleeder at a time in the rear and let the fluid run till you see no bubbles.

Be sure to keep the master full.

Gravity bleeding usually works quite well.
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PostPosted: Mon Apr 25, 2016 7:14 am    Post subject: Re: Bleeding brakes - nothing but bubbles in the rear Reply with quote

Is the pedal firm?

Many times air is introduced by the threads of the bleeder screw. Sucks air into the hose like a carburetor sucks fuel into the venturi.

Since the air is being introduced after the system, you may not have a problem at all.
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PostPosted: Mon Apr 25, 2016 7:17 am    Post subject: Re: Bleeding brakes - nothing but bubbles in the rear Reply with quote

Butcher wrote:
Is the pedal firm?

Many times air is introduced by the threads of the bleeder screw. Sucks air into the hose like a carburetor sucks fuel into the venturi.

Since the air is being introduced after the system, you may not have a problem at all.



Edit: I assumed you were vacuum bleeding, since you pressure bleeding at the MC this doesn't apply. Embarassed
I second this suggestion. Put some grease around the bleeder screw threads. If the bubbles stop, that is your source. No need to keep bleeding if you have a firm pedal.
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PostPosted: Mon Apr 25, 2016 7:19 am    Post subject: Re: Bleeding brakes - nothing but bubbles in the rear Reply with quote

Not really.
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PostPosted: Mon Apr 25, 2016 9:05 am    Post subject: Re: Bleeding brakes - nothing but bubbles in the rear Reply with quote

MidwestDrifter wrote:
Butcher wrote:
Is the pedal firm?

Many times air is introduced by the threads of the bleeder screw. Sucks air into the hose like a carburetor sucks fuel into the venturi.

Since the air is being introduced after the system, you may not have a problem at all.


I second this suggestion. Put some grease around the bleeder screw threads. If the bubbles stop, that is your source. No need to keep bleeding if you have a firm pedal.



if you do grease the bleeder screws (not a bad idea) be sure it is brake system compatible grease. standard greases can harm the brake rubber bits even small amount in the fluid can be disasterous. so brake compatible grease only The other upside with grease is no mkre rusted on screws!
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PostPosted: Mon Apr 25, 2016 9:58 am    Post subject: Re: Bleeding brakes - nothing but bubbles in the rear Reply with quote

Brake fluid can get itself aerated especially when using pressure bleeders. At a more ambient pressure the bubbles expand an make fluid look more like foam. It may take quite a while to consolidate the fluid. I have even found that it takes several days for air to make real bubbles for a good bleeding.

After you think your brakes are bleed and working well but a little soft, you should bleed them again.
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PostPosted: Mon Apr 25, 2016 10:46 am    Post subject: Re: Bleeding brakes - nothing but bubbles in the rear Reply with quote

Butcher wrote:
Is the pedal firm?

Many times air is introduced by the threads of the bleeder screw. Sucks air into the hose like a carburetor sucks fuel into the venturi.

Since the air is being introduced after the system, you may not have a problem at all.


Very common. When I change brake fluid on my cars, I have the bleeder very loose to move lots of fluid through. I often see bubbles from the threads in this mode. Once I get clean fluid coming out, I tighten the bleeder until I am working to push fluid out. The bubbles stop in this mode. You aren't very far from final tightening, maybe 1/4 turn or so.

If you replace all your bleeders with new ones when you replace the fluid every 2 years, you will never snap them off, btw.
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PostPosted: Mon Apr 25, 2016 11:58 am    Post subject: Re: Bleeding brakes - nothing but bubbles in the rear Reply with quote

30 years of pressure bleeding, this is a common issue. The first time I ran into this, I sure wasted a lot of time and fluid. I could be wrong with the explanation, but it makes sense to me and I was able to get along with other life problems.

So, if the pedal is firm, I would not worry about the bubbles.
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PostPosted: Mon Apr 25, 2016 1:29 pm    Post subject: Re: Bleeding brakes - nothing but bubbles in the rear Reply with quote

Butcher wrote:
30 years of pressure bleeding, this is a common issue. The first time I ran into this, I sure wasted a lot of time and fluid. I could be wrong with the explanation, but it makes sense to me and I was able to get along with other life problems.

So, if the pedal is firm, I would not worry about the bubbles.


Been Working on cars since the 1960's....... First time I ever heard anyone state that bubbles in your brake fluid are OK ..... Shocked

Bubbles are air pockets, air pockets contain moisture and the air can also compress.

There is absolutely no way that air bubbles in a brake line are OK ........

The brakes may work, and they may even stop the vehicle but to say that they are of no concern and to not worry? ......... Rolling Eyes

Find out why you have bubbles and fix it, ignore the advice to not worry.....

Dave
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PostPosted: Mon Apr 25, 2016 1:50 pm    Post subject: Re: Bleeding brakes - nothing but bubbles in the rear Reply with quote

I believe if you read everything I said, it's not what you are implying.

I said that the air may be coming from the bleeder valve threads. The air gets sucked into the threads and out the bleed hose. Never did I say that the air was trapped in the brake system.

I'm certainly not the first to have seen this and it is very common for those that do this for a living. There may be a better explanation of why there is air the bleeder hose, but I have yet to hear any on why this happens [short of air being in the system]. You can only flush so much brake fluid through the system before you have to start to figure out where the air is coming from. With a pressure bleeder you are pushing fluid [not shaken up brake fluid where they could be air in it]. Where is the air going to enter the system? If air could enter a pressurized system you could patent that and never have a flat tire again.

You can always seal the threads like someone has already mentioned and see what happens. I'm not there, but I can tell you as much bubbles that I have seen, the brake pedal would be spongy. Since it is not, that is my reasoning on where the air is coming from.

I believe you have work with/for Mercedes. If you have, then you may be familiar with the SBC system. That system has the same issues with bubbles coming out of the hose. That system also has a way of testing to see if there is air in the system. I can assure you, all my SBC repairs passed the air test and many had bubbles in the bleeder hose.

I'm always willing to say I'm wrong. I will always agree, I may not be saying correctly. I did Google search it and it is common, very common.
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PostPosted: Mon Apr 25, 2016 1:53 pm    Post subject: Re: Bleeding brakes - nothing but bubbles in the rear Reply with quote

brakes are a hydraulic system, air in a hydraulic system is bad, it works against the mechanism of the system and is contrary to the entire need to use such a system...


bubbles in your break fluid is NOT ok... do it the old fashion way sans the power bleeder... It may very well just becoming off the bleeder screws.


*EDIT:* Not calling anyone out, or saying anyone said anything to the contrary... Just making it clear that bubbles in your break fluid is bad.
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PostPosted: Mon Apr 25, 2016 1:54 pm    Post subject: Re: Bleeding brakes - nothing but bubbles in the rear Reply with quote

he is saying that the bubbles are a transient epiphenomenon and do not persist in the fluid once the screws are closed
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PostPosted: Mon Apr 25, 2016 2:18 pm    Post subject: Re: Bleeding brakes - nothing but bubbles in the rear Reply with quote

I agree with what Butcher is saying here. The air that is being seen is not in the brake lines. It is being introduced at the threads of the bleeder or could even be introduced between the bleeder and the fitting or hose that it attached to it. I've seen it happen many, many times.

Of course, we are assuming there are no leaks and no other fault that would allow air into the system.
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PostPosted: Mon Apr 25, 2016 2:19 pm    Post subject: Re: Bleeding brakes - nothing but bubbles in the rear Reply with quote

When vacuum bleeding very dense fluids it is possible to get bubbles of vacuum. IE bubles with no air in them. When the pump/vacuum unit is removed the bubbles collapse. In a pump this is often called cavitation.

I doubt that is happen in this case though. Confused
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PostPosted: Mon Apr 25, 2016 2:54 pm    Post subject: Re: Bleeding brakes - nothing but bubbles in the rear Reply with quote

MidwestDrifter wrote:
When vacuum bleeding very dense fluids it is possible to get bubbles of vacuum. IE bubles with no air in them. When the pump/vacuum unit is removed the bubbles collapse. In a pump this is often called cavitation.

I doubt that is happen in this case though. Confused


He is using a Motive pressure bleeder. Same as what I use. It pressurizes the reservoir so no possible vacuum issues.

My vote is that the bubbles are at the bleeder fitting somehow, BTDT.
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PostPosted: Mon Apr 25, 2016 3:00 pm    Post subject: Re: Bleeding brakes - nothing but bubbles in the rear Reply with quote

My Mercedes days were pre pressure bleeders. Up, down, up down, up, down, up......... Down.

While that may be what is happening, I'd be darn sure that I knew for sure before driving off.

Not just say "it's all good, some guy on the internet said it was."

Err on the side of caution.


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PostPosted: Mon Apr 25, 2016 3:28 pm    Post subject: Re: Bleeding brakes - nothing but bubbles in the rear Reply with quote

djkeev wrote:


While that may be what is happening, I'd be darn sure that I knew for sure before driving off.

Not just say "it's all good, some guy on the internet said it was."

Err on the side of caution.


Dave



Yes, of course! But he also said "The brake and clutch pedals feel pretty firm". If that is the case, there is no way it has massive amounts of air in the system. Yes, there could be a minor leak. Something to keep an eye out for, but all indications seem to point to air being introduced from outside the system with the most likely culprit being around the nipple while it is loose.
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PostPosted: Mon Apr 25, 2016 3:45 pm    Post subject: Re: Bleeding brakes - nothing but bubbles in the rear Reply with quote

I third Butcher's assertion that very likely the air is being sucked in thru the bleeder's thread. I found this out in hi school. I put a piece of arts putty around the nipple and all is good. Very Happy
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