Braking/Brake Servo Problem

[phatmendus]

Hi,

Does anybody have any ideas?..... recently my brakes have been not working as they should, it is tempremental and gives the effect that the servo is only working intermitently. when i press the pedel sometimes it is fine, and then othertimes it is as if the servo is not assisting the breaking at all.

I have checked all the vacuum lines for leaks, checked the system is pulling and maintaining a vacuum and checked the one way valves are working as they should. All the listed items were working fine, and the servo unit seems to be be holding the vacuum without a problem.

It is the delphi replacment system that was fitted about 5 years ago and I have never had any problems with it before. Can anyone offer ay advise on what they think it could be? Perhaps something linked to the hydraulics rather than then vacuum system? As I say sometimes it works fine, but others not at all and I have to step on the brakes very hard!

many thanks in advance

Simon

[leifanten]

Does it depend on engine RPM whether it works or not?


I was thinking that you may have a check valve that is a little slow on low vacuum, but opens up at higher than design vaccum. If so, the brakes would work well at idle and not well under wide open throttle.

Did you check the main diaphragms of the boosters?

[phatmendus]

It seems to be worse if anything at low revs, but not a great deal of difference really, it works usually with the first braking then doesnt after that for a bit (which i thought suggested vacuum)

where would i find the main diaphragm for the booster you talk about?

thanks

Simon

[leifanten]

The diaphragm is containded inside the main canister of the vacuum unit. It is the "amplifier" of the system. But if it works the first time and not thereafter I would start checking the one way check valve in the system. It sounds like it is binding in either open or shut position (I cant get my head around which way it is mounted right now )

[collins_dan]

It might be a poor seal in the master cylinder that is allowing brake fluid back into the reservoir when brakes are applied slowly. Try applying the brake very slowly just sitting in the garage and have someone look at the reservoir to see if the level rises as you depress the brakes. If it does, then you need a master cylinder rebuild kit. I have a dual system on my federal S4 and the front seal was installed incorrectly, so the brakes worked fine on hard stops, but on slow stops it was pushing all the fluid back into the reservoir, so when I went to stop the next time, I had bad pressure.

Dan

[phatmendus]

Yeh I thought it may be the one way valve sticking as well, and checked the one that plugs into the vacuum unit from the hose coming out of the inlet manifold, and i also checked the valve in line on the hose from the manifold.

Are there any other valves I need to be aware of or just those two?

[collins_dan]

Actually, I just re-read your problem and if you are having to apply really hard on occasion, its probably not a bad seal. It does sound like the servo, which I don't have, so can't help.

Dan

[leifanten]

those are the only valves on those lines that I can think of. Sound like a rebuild of the unit itself is next in line.

[phatmendus]

yeh, could be the case! dam it.

I was just unsure whether some hydraulic problem would give the same indications. Ill check out the other tip about the master cylinder but doesnt sound like the same symptoms. rebuild here we come!