Help me PLEASE!!!!!!!

The car was originally a drum rear brake car. Changing the master cylinder should have gotten you one for a rear disc brake setup. However, some odd things to check, make sure there is not a drum brake residual valve in there somewhere. These sometimes are in the m/c rear brake outlet fitting. Most likely that function was done by the original proportioning valve.

You say theres no proportioning valve, but say you have an adjustable one on the rear brakes. NOt sure what you mean. The original setup had a combination valve in there, with the brake failure warning light valve/ switch, and a proportioning valve for the rear, in the "combination valve" that is under the master cylinder. If you added another, you now have two.

Had a Scout with this problem, and locked up brakes once, and the m/c pushrod turned out to be too long. It would not let the m/c piston come back far enough to open the return passage, and the brakes stuck on, with the pedal feeling hard.

I can't easily suggest a fix, other than to get a set of parts that's proven. It could be that the m/c diameter is too big, losing advantage to the larger rear disc calipers.

The booster is supposed to give a harder pedal feel when theres no vacuum. Its a sign of failure when it has high pedal effort when the engine is running. Also make sure the vacuum check valve in the booster is ok. Its the plastic thing the vacuum hose goes onto.

I'm (obviously) not a brake expert, but some of the above should help.