940 non-turbo

Hi all,

I wrote many threads back about experiencing a pulsating rpm after I brake hard and sudden, coming to a stop. Just to repeat the information, I tried the brake servo test by pumping several times on the brake with the engine off to rid the vacuum. Then start the engine and could feel the brake pedal depressed. Hold the pedal there at the last held position, off the engine, and then start again. The pedal depressed further in. Did this sequence several times and at each start of the engine, I could feel the pedal sinking in further. So does this point to a faulty brake booster or is it something else?

Please can someone respond because I think the situation is getting worse because the pulsating rpm occured today while at a traffic jam with me holding the brake and not on a sudden hard braking action.

Thanks for your response in advance.

Laozong

I'm not sure I fully understand workings of the brake booster, so I hope someone else can pipe in here.

The fact that your engine runs OK without depressing the brake, suggests that the vacuum hose to the booster check valve, and the booster's vacuum reserviour seal are both OK. Pressing the brake then releases this vacuum into another section of the booster to provide a mechanical advantage in depressing the master cylinder piston. I'm guessing that your leak is in this other section of the booster.

If your problem was just a leaking check valve, ie. the booster didn't hold a vacuum reserve, then you would get your problem when you first depressed the brake, but after a period of time, the vacuum would eventually build up and no more air would be drawn from the booster. Is this what you've noticed, or does the engine continue to have a problem for as long as the brake pedal is depressed? (Hopefully your engine doesn't die before you can make this judgement).

If the problem goes away while holding down the pedal, maybe it is the check valve. If it never does, I would think the problem is the booster itself.


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David Armstrong - '86 240(350k km?), '93 940T(270k km), '89 240(parts source for others) near Toronto