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1981 VW Rabbit brakes having issues

I am having issues with my brakes on a 1981 VW Rabbit. It has a 1.9turbo diesel engine, and nothing is necessarily stock, but I’ve narrowed down some basic parameters:

The booster push rod is the major adjustment that I know makes changes. 1/4 turn one way, and the brake pedal goes almost to the floor, 1/4 turn the other way (out) and the pedal feels great, brakes work wonderfully with great power assist, but the brakes drag (I can hear it when I push it and the wheels heat up). When the brakes drag, they only drag when there is vacuum pressure. If I take the vacuum line off or turn the engine off while rolling and pump out the pressure, the drag goes away completely.

Is the booster push rod adjustment supposed to be so sensitive? If so, why? It seems like pushing down slightly on the pedal would be the exact same thing as turning the adjustment out… I am missing something in the theory of operation, and could really use some help understanding this one.

Things I have done: I have a brand new, high end master cylinder, plenty of fluid and the level stays good, vacuum pressure is solid. I have pumped the brakes, and it did seem to help once, but it has started to seem more like a random item…

Thanks,

Yup, you are missing something in the theory of operation. When the rod is fully extended, foot off the brake, the vacuum is applied to both the front and back of the diaphragm equally. When you push the rod in, even just a little, the rear of the diaphragm is vented to ambient and the vacuum applied to the front only. If you adjust the rod such that it’s never fully extended, the vacuum will stay o the front of the diaphragm only, the back will stay bented, and your brakes will drag.

What is the fix SAM? proper rod adjustment?

So, I got my girlfriend to help me pump the brakes. Pumped the rear two drums, and bubbles just kept showing up. I went through a lot of fluid, and bubbles just kept coming, and then I didn’t have any more back up fluid, so I took it for a drive. The brakes are like new! So: what’s with the air? I had pumped them out before, but it was a while ago… The fluid level never went down, so a leak doesn’t seem likely…??? I spent three days on this, and it would be great if I can take some theory away from it.

Next time bench bleed the master cylinder?

Sometimes a new master cylinder has to be bench bled to get all the air out. Did the instructions that came with the new unit say to bench bleed it before installing on the car?

If you’ve bled the master cylinder correctly, it may be you didn’t bleed the four wheels completely. You could try again. I’ve had problems w/VW Rabbit brake bleeding. Hard to get all the air out. Probably what I’d do is take my car to a inde VW specialist mechanic and have them do a pressure bleed. They bleed all four wheels and once and is very good at getting all the air out of the system. Works much better than manual bleeding in my experience. You can have the mechanic check the adjustment on the booster too and get a two-fer.