My brother has a 1985 300SD and the brakes started acting up recently. It started with squishy brakes...we replaced the 2 front rotors and then bled the brakes...the fluid seemed rather watery. The car sat a long time before we bought it and started driving it regularily. The brake pedal seems hard now...and you really have to give it hell to make it stop at all. I think it may be an obstruction in the line...I had a beetle that gave a similar problem, it was a rusted flex line. Anyways, its undriveable cause it cant stop in an emergency. So what could it be? How could I condemn parts like master cylinders or boosters? What should I look for to find the problem.