Brakes fade when I park!!

[Green Machine]

This is a weird one. My '80 300SD brakes perfecly 99.9% of the time. But, if I'm backing into a tight spot to parallel park (most spots are tight in an SD), my brakes fade almost completely, giving me quite a scare as I nudge the guy behind me. Anybody had any similar experience? Am I pinching a line somewhere or what? Once I pull back out onto the road, my brakes return to normal immediately. What the hey!?

[diesel lover]

It only does it while ideling? If that is the case I would say there might be something wrong with your vaccume pump or something in the vaccume system. But I am no expert. I dont think it would be a faulty proportioning valve.

There should be enough reserve in the servo to give 3 or 4 applications of brakes after the engine is stopped, if the servo is working at higher revs but fails as soon as it idles or low revs (when parking) I would suspect there is a leak in the servo unit.

[Green Machine]

Thanks for your responses, guys - The brakes seem to be fine at idle when moving in a straight line (forward or backward); the only time I notice a nearly complete loss of braking is when backing into a parallel parking spot, when the front wheels are turned nearly to a left lock - the brakes will then remain faded until I pull back out and give it a bit of fuel. This seems to be consistent with what you were saying, desmondo - and the vacuum idea seems a good bet too, d.l. - I just can't understand why it only seems to be brought on in a tight turn situation. Any relationship to vacuum assist in the power steering, maybe?

When you say "fade", do you mean the pedal goes to the floor, or do you mean the pedal gets very firm and is hard to press? The former symptom is indicative of a hydraulic problem (e.g., master cylinder), the latter a vacuum problem (bad vacuum servo, vacuum leak).

[MikeTangas]

Since it only happens when turning the wheel while backing, when was the last time the rubber brake lines were replaced? Possible they could be breaking down internally and the added stress of turning the wheel prevents fluid flow.

[Green Machine]

Paul - when I say 'fade', i mean hard pedal, so - vacuum problem, right? There seem to be no other vacuum-related problems with the car though - the locks work fine, tranny shifts nicely etc. Mike, I'm not sure when the brake lines were done, if ever - I've only had the car for the last 6 or 7k miles. Is there a quick visual inspection that can determine their condition? Thanks again for your help.

It's impossible to determine the internal condition of these hoses visually. However, this problem usually manifests itself through a failure of the brakes to release promptly, not in a failure of activation. In any event, the rear hoses would not flex in reaction to hard steering at very low speeds, so the back brakes should still be operative and provide at least a slight sensation of braking. Given the age of your car, I would replace them anyway, coupled with a flush of the brake lines.

However, I suspect that your real problem lies on the vacuum side of the system. I would suspect that the vacuum accumulator is not doing it's job, and that cutting the steering over hard at low rpm causes the power steering system to burden the engine, lowering the rpm further, and therefore, production of engine vacuum. The accumulator should be pitching in at this point to allow normal braking assist during this brief period. Apparently, it is not. In terms of vacuum, your brake servo is living hand-to-mouth, i.e., it is operating solely on whatever vacuum is being generated at any given moment by the engine. Review Desmondo's post.

I suggest a test: FIRST: Ensure that the parking brake is in good working order. Then, in an open, level, straight, deserted and safe area, drive up to about 20 mph, shift into Neutral, turn off the engine, and begin pumping the brakes. If you get 3-4 pumping actions before the pedal firms up, vacuum might be ok, if not, vacuum problem definitely. Just remember, you will lose power steering assist during this test as well, so be safe.