V8-4.7L,Hard brake pedal and rear brakes locking up.

I had an issue with a spongy pedal so I replaced my rear wheel cylinders as it felt like my back brakes were grabby. I got the new ones on and had absolutely no brake pedal. Trying to fix the problem I put a new master cylinder, brake booster, a brake line I found was leaking, a booster check valve and bled the brakes a whole bunch of times. My pedal is now hard and it seems like the front brakes aren't doing anything and the rear brakes want to lock up unless you barely tap the pedal. I've since dropped it off at the dealership but would like to know some possible causes of the proble