M38 Master Cylinder

[Richard Whelan]

Anyone out there know how to diagnose a master cylinder problem on my M38.
It is new. The piston would not return properly when I first bench tested it on my vice. It was well lubricated but would not work properly. So I honed out the bore until the piston moved freely. Bench test it again with brake fluid and no leaks were found so I installed it and started to bleed my brakes.

Now here is where the problem begins. When I bleed a wheel cylinder the piston takes about 2 minutes to return fully to the rear. I think there is some sort of vacuum problem but do not where to look. I think it is still in the master cylinder but not sure what to do about it. I've checked the intake and bypass holes and they are clear.

Note. When the piston finally returns fully the master cylinder functions correctly but the brakes are still spongy. So with the problem when bleeding a wheel cylinder this prevents me from removing all the air.

Completely confused