What if????? What if you have a master cylinder that is
for a disc front and a drum rear combo? You would be
severely lacking in pressure on the back wheels and,
since the calipers take more fluid than do drums, you
would over tax the master cylinder's ability to deliver
fluid.

Do you know that you have a disc/disc master cylinder?

Also, the power brake push rod is adjusted so there is
just a smidgen (a technical term) of play when the brakes
are released. If there is no play, the master cylinder may not
fully return to the released position.

mike in tucson