help diagnose a brake problem

[DavidF]

The brakes on a 63 220S are doing something odd. They stop working. Then they start working.

This has happened three times now. I find the brake fluid chamber empty. I refill it, pump up the brakes, drive carefully home. No fluid leaks on the drive home. In the morning I bleed the lines and the brakes work as promised for several months.

The first two times there was no puddle under the car. The third time I found some fluid -- but not more than 4 ounces -- dribbled around under the left front corner. The area under and immediately behind the brake booster is very wet. The master cylinder and all the fittings into the wheels are dry. My guess is it is the booster. But why do they work some times. I don't believe brakes can heal themselves.

[newtodiesel]

I have a 123, not sure but your model could be similar.
According to what I have read here, there is a seal between the booster and the brake resevoir, could be a cheap easy fix. I have been told also if you r not finding fluid often, it could be going inside the booster. A seal that costs less than a few bucks might be worth the trouble of taking it apart and putting it back together. Many here might say not worth the safety just change out the booster, your call. I know my pocket is very light these days

[Pooka]

To expand upon what Newtodiesel just said.....

If the seal at the end of your master cylinder is weak then suction from the brake booster could be pulling the fluid out of the master cylinder. How can it do this? Because you would also have a leak in the diaphragm inside the booster which would allow the engine vacuum to not only pull down on the diaphragm but also to pull down on the back of your master cylinder.

One way to test this is to remove the master cylinder from the brake booster and start the car. If you feel vacuum at the point where the master cylinder bolts up to the booster then you have a diaphragm leak and will need to replace the booster.

Yeah, I know.... That's a lot of work. But those brake boosters are not cheap, so you need to know that you need one before spending the money to replace the one you have.

You could also plug off the vacuum booster line. You would then have unassisted brakes, but if the fluid loss stopped you would know you were one step closer to finding the trouble.

[BRM]

while your are in there you should check the condition of your rubber lines and the metal lines. if your metal lines are old you should consider replacing them. they rust from the inside out.