Stubborn air bubbles in brake lines - PeachParts Mercedes-Benz Forum

JHZR2 · #1 11-17-2003, 09:12 AM

Hi,

First off, the car I am having trouble with is a 91 BMW 318i. I recently replaced the ABS pump with a working used unit, and of course have to get all the air out.

I bought a pressure bleeder for this purpose, and went at it, putting 15psi on the system. The first time I bled, a lot of air came out. I then drive it some, and the pedal travel is definitely longer than before. It stops fine, etc., its just that I have to push farther before I get to the point where the brakes really engage and the pedal is rock hard beyond there.

I re-bled, but the pedal feels exactly the same. I got one timy bubble out of one of the calipers, but nothing anywhere else.

So I need to do something a little more stringent.

Would it be OK to use my pressure bleeder, AND have someone pump the brakes, like as doing the old fashioned bleeding procedure? This way the MC will always have fluid being forced into it, and the level will remain nearly constant. Plus, there will be no chance of air coming back up and in, becasue there will always be the small flow from the 15psi coming from the pressure bleeder.

Also, what if I turned the car on while pressure bleeding? If we just pushed the pedal a very little bit, a LOT of fluid ought to come out and force any bubbles through, right?

Either way I think Id use a brake pedal stop so we couldnt get it all the way down to the floor. I want a high pressure and high flowrate, but I dont know that I want to be going full length on the pedal stroke (thats part of the reason why I bought a pressure bleeder... they said that pushing the pedal full stroke is bad).

Ill be changing to ATE super blue this next flush (I wanted to flush a lot to get whatever was in the replacement ABS pump out, so I started with some leftover ATE DOT4 SL fluid), and so if the old stuff comes out fast, as in doing it with the car on, etc., it just makes my job easier...

Any comments?

Thanks,

JMH

#2 11-17-2003, 09:38 AM

ABS systems can be a PITA to bleed properly. I have an Expedition with ABS, and the factory recommended bleeding procedure involves hooking up a $2,000 piece of equipment called a NGS Star Tester to work the ABS solenoids while bleeding in order to evacuate air from around the solenoids. I wonder if BMW employs a similar method.

BTW, is DOT 4 recommended for this vehicle?

Also, if the brake hoses are original, now is a good time to replace them.

JHZR2 · #3 11-17-2003, 09:41 AM

yes, BMW has used DOT4 since 1983 or so...

Ive heard about the need of those systems, but apparently it is doable in a BMW by just bleeding... The shop manual apparently says so.

JMH