Quote:
Originally Posted by john walker's workshop
made in czech republic now, not that that really matters. there's been a couple of cars lately with a fresh MC, which after a thorough bleeding, PMB rebuilt calipers and new hoses, still had a soft pedal at the top 1/2 of the travel. basically the rear brakes are not getting enough pressure. the fronts lock up on a hard stop. 80 and 82 SCs. all the dual stage MCs have a kind of hard "stop" that you can feel if you press hard enough, which is just the front system. usually you don't get that far because the brakes respond well way before you reach that point. the pedal on these cars is rather "soft" at the top 1/2 of travel. both cars got another MC after the first one felt weird, but the problem remains. lots of bleeding by pressure, foot and vacuum to no avail. not an air bubble to be seen. tried pumping out one piston at a time, prying it in just enough to slip the pad in, tried some road time over a few days and a rebleed. running out of ideas. everything points to a bad batch of MCs. anyone else had this problem lately?
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It sounds like air in the m/c or possible a leaking seal on the primary piston
these are simple systems
the pedal pushes a rod which mechanically pushes the primary piston, in a nominally functioning system hydraulic pressure in the primary(front brake) chamber pushes the secondary pistons pressurizing the secondary(rear brake) chamber, springs and hyd back pressure return the system to neutral
if there is air in the primary chamber it can allow the primary piston to compress the spring between the primary and secondary piston causing the two to connect mechanically rather then hydraulically
Just a guess here