Brake Pedal Fade Away

[merkur5271]

I am working on a 1997 C-3500 dual wheel truck brake system (2wd, R engine, Hydro-boost). The left rear wheel cylinder blew out and poured out massive amounts of brake fluid. After replacing the rear brakes and wheel cylinders and having the drums turned, along with a new master cylinder (M/cylinder was due to age and 178,000 miles) and replaced the proportioning valve due to it's leaking out under the rubber dust cap..bleeding out the brake system including the automated bleed of the ABS unit results in a high, firm pedal.... but after 4 seconds, the pedal goes to the floor like a master cylinder is by-passing. So, I bought a A/C Delco master cylinder and repeated the process..... same exact problem. Am I dealing with a bad ABS hydraulic unit?

[ChevyTech77]

I am working on a 1997 C-3500 dual wheel truck brake system (2wd, R engine, Hydro-boost). The left rear wheel cylinder blew out and poured out massive amounts of brake fluid. After replacing the rear brakes and wheel cylinders and having the drums turned, along with a new master cylinder (M/cylinder was due to age and 178,000 miles) and replaced the proportioning valve due to it's leaking out under the rubber dust cap..bleeding out the brake system including the automated bleed of the ABS unit results in a high, firm pedal.... but after 4 seconds, the pedal goes to the floor like a master cylinder is by-passing. So, I bought a A/C Delco master cylinder and repeated the process..... same exact problem. Am I dealing with a bad ABS hydraulic unit?

Yes, sounds like the hydraulic unit (bpmv) is bypassing internally.