1996 GMC C1500 brake pedal leaking down

If you have a brake pedal that slowly goes to the floor, you either have a bad MC or a leak. Since you replaced the master cylender, there has to be a leak. Have you checked rear wheel cylenders for seapage? Have you checked all rubber brake hoses and steel lines for leaks? Are the rear brakes properly adjusted so as to not over extend the wheel cyenders?

Let me know.

Hi hooterman10, Welcome to 2carpros and TY for the donation.

If there's no leakage anywhere could be the ABS unit or the master cylinder again is leaking internally thru seals/cups-investigate

Try both ways if it still sinking and no leak- rear properly adjusted I'll lean towards the MC

To confrim the brake booster is working -pump the brake a few times hold it in and start the vehicle the pedal should move down a lil.

Sounds like the master cylinder is gone if you are properly bleeding the brakes

Replace the master cylinder

Okay lets try this bleed the MC and then the ABS unit and all the wheels from RR/LR/RF/LF-

No its not common -but could happened

Not common, but it does happen. If there are no leaks, fluid has to be bypassing the seals in the master cylender. You may want to try a different brand.