Vacuum servo - air leak across diaphragm

[devinaandethel]

Car Series 2, 2+2, 1969 LHD.
I noted recently that just after the engine was switched off the brake vacuum dropped quite quickly. Eventually I found three leaks, the check valve, the joint between the reaction valve and the master cylinder body and the rubber pipe to the brake servo. To aid finding the leaks I took the components off the car; using a 12v small vacuum pump to check the servo for leaks I connected the pump to both sides of the pneumatic diaphragm. A good vacuum was achieved and when the pum was switched off the vacuum remained steady, no drop.

However when I connected the pump to just the front side of the diaphragm (i.e. nearest to the hydraulic cylinder) the pump was unable to establish any vacuum UNTIL I blocked the pipe to the rear volume. So clearly there is a "leak" across the diaphragm, checking the latter visually I am certain that it is not punctured, so conclude that the "leak" is at the centre, where the rubber fits over a boss in the plastic support.

I would have thought that for the servo to operate efficiently there ought not to be any air flow between the 2 pneumatic chambers.

So my questions are thus, is this normal, has any other member had a similar experience and how does one cure the leak?

[christopher storey]

I don't think this is indicative of anything amiss. The point is that in normal working, when you press the brake pedal , not only is the hydraulic shuttle moved forward , but ambient pressure air is admitted to the rear chamber which ( because of the pressure differential between the fron and rear chambers ) pushes forward on the phenolic piston in the servo thus assisting the pedal pressure. A read of section L.X.S in the workshop manual, and figs 1 and 2 on pages L.X.S 2 and 3 will show you what is happening

A more reliable guide to the servo state is to start the engine to allow vacuum to build , then stop it. When you press the pedal and then release it , you should hear a knock from the front floor which is the sound of the diaphragm operating. With each sequential press it should diminish in volume and intensity until the vacuum is exhausted

[devinaandethel]

Thank you for your reply. I agree totally with your description of the operation, but my concern is that if there is a leak through the centre of the diaphragm (where the rubber sits in a circular groove) then when air is admitted to the rear chamber it will pass into the front chamber and the pressure difference between the two sides of the diaphragm will be reduced and the pressure amplification (in the ratio of the areas of the pneumatic and hydraulic pistons) will be reduced thus affecting braking force.

I must admit that when previously (ie several years ago) overhauled the braking system I just used the servo repair kit as instructed, replacing it and the master cylinder without any vacuum testing. The testing I have recently done was to find why the vacuum dropped quickly when the engine was switched off. Thus the car may well have been operating happily with this "leak" across the diaphragm.

[MarekH]

Two points:-
1/ You didn't report a horribly heavy brake pedal, so one presumes the booster was working. So long as the engine is running and you don't have the throttle mostly (or wide) open at low rpm or completely open at higher rpm, any loss of vacuum will be continuously replenished.
2/ If there were a leak during normal running, then this forms part of the air entering the cylinders and it's absence would manifest as a drop in idle speed if you were to eliminate that leak. To test that, get your car happily warmed up and idling, check the idle speed and then seal off both sides of the booster as you did earlier. Any leakage across the diaphragm will now be zero and less air will reach the cylinders - the ide speed will drop. If no significant air leakage across the diaphragm exists, the idle speed will be unchanged.

kind regards
Marek