[BMERDOC]

If you have long pedal travel and the rear lock up first I'd say the rear cup in the master cylinder is bad. The front reservoir is for the rear drums and the rear is for the front discs. The cup in the master for the front brakes is bad. This is the fundamental reason for a split system so that all the brakes aren't lost if one segment loses pressure. Replace the master cylinder. If the rears still like to lock up somewhat prematurely then loosen up on the rear adjustment until they do. You should be able to spin them and have the drum still want to spin on its own for almost a full revolution. Its an old tried and true method for adjusting the rears.