4 replies to this topic

[bc130]

I have a 1989 mini and i have just hooked up the brake servo after rebuilding the car. During the rebuild i painted the servo and master cylinder.I didnt split the cylinder from the servo and hung the whole lot up to dry after spraying.A few days later i took it down to discover that there was brake fluid after seeping from the cylinder into the servo and out through the air filter on the back of the servo.I ignored this issue and after putting in the servo today, i filled the reservoir but quickly discovered a major leak around one of the connections on the pressure reducer and the reservoir soon emptied. I also noticed a drop of fluid seeping from the joint between the servo and the master cylinder.On pushing the brake pedal in the car, it goes down but doesnt come back up. The question i have is should the brake pedal return to its original position after removing your foot when the car is turned off and the brake fluid is empty. Im not sure how the servo works and dont know what provides the back pressure in it to return the pedal to its original position. I also presume that my master cylinder is leaking when there is fluid getting into the servo. Any help would be much appreciated.Thanks.

[dklawson]

I suspect you have a perforated diaphragm in the servo and that the collected brake fluid is somehow hindering the return of the pedal.

Succinctly, the big housing of the servo is a chamber connected to the intake manifold. Vacuum from the engine is used by the servo to multiply the force you apply to the brake pedal. That results in less pedal effort being required to stop the car.

If the rubber diaphragm inside the servo fails, brake fluid can be sucked out of the reservoir and eventually be burned in the engine. If you have noticed any white acrid smoke in your car's exhaust recently, this would confirm the diaphragm has failed.

[Ethel]

What did you hang it up by? Sounds like you may have pulled the piston from the master cylinder, unbolt it from the servo & have a ganders.

It's actually a very simple device, there are a couple of simple valves, not unlike sink plugs, around the push rod you can see sticking out the back and linked to the pedal. When you push the pedal you open the valve that lets it breath through the filter you mentioned and close the other that connects the front & back of the vacuum can on either side of the diaphragm. That lets atmospheric pressure push on the back while the vacuum is maintained in front of the diaphragm. If the diaphragm catches up with where you pushed the valves they'll close off the rear vac chamber from the atmosphere to control how hard the servo pushes and if you take your foot off the front valve will open so vacuum is returned to both sides of the diaphragm and the servo stops pushing.

Afterthought, you didn't have the fluid reservoir off while you were painting did you?

[bc130]

DKLAWSON: If the diaphram isnt punctured, should there be a seal preventing the fluid from flowing through the servo. I should have said also that i have been restoring the car over the last year and it had been parked up for about 6 years before hand so i am only reassembling the brakes now so i havnt had to a chance to notice any white smoke. The fluid only came through the servo when it was hanging up. It now drops around the joint between the servo and the master cylinder (not muh do).I will more than likely replace the cylinder as it is probably corroded inside anyway but im not sure if the servo will be ok or not.

ETHEL: I had the unit hanging with a bit of wire that i had put through the gap between the master cylinder and the reservoir. I may also have had it hanging the other way around at some stage i.e. it was suspended from the push rod.No, i never took the reservoir off.If its only something as simple as pulling the piston out of the reservoir, would there be any fear that i would have over stretched the diaphram in the servo then.

Thanks for the replies.

[dklawson]

The brake fluid should never really reach the diaphragm but when there is a leak it seems to find its way there and collect inside. Not everyone wants to rebuild a brake servo but if you are planning on replacing it anyway, I would at least carefully open it up in a clean spot and see what you can determine about the seals and the actuator rod that will be inside. It could be as simple as a failed seal. If it is anything more serious, it may prove to be less expensive and more reassuring/safe to buy a replacement rather than rebuild what you have.