Did this happen all of a sudden or gradually. I had the same problem when I bought my 86 560sl. After flushing out the brake fluid all is fine. If flushing by pumping the brake pedal try not to push the pedal to the floor or you will be replacing the master cylinder. They build up a ridge that will tear the seals if they are pushed over the ridge.

The reason it seems to happen after driving a few miles is old brake fluid will have moisture. After you use the brakes a few times the water will boil and you will then have air bubbles that do not compress at the same rate as the fluid. Pumping the brakes overcomes this problem but not for long.

I let mine go for a couple of days but I do not recommend it. Changing the fluid is alot cheaper the repairing the front end.

I would take your email address out of your post unless you want spam. Add it to your profile instead if you want it available to members of this great site.