1998 chev c 1500 soft/no brakes. Replaced master cyl 4 times, booster, abs unit, all rubber lines, rear wheel cylinders and shoes, front calipers and pads, still no brakes. In desperation bypassed abs with proportioning valve, in all bled brakes dozens of times including vac bleed and dealer scan for abs. I have worked on brakes for over 40 yrs. And this one has stumped me.

Thoughts:

Did you bench bleed the master cylinder, prior to installation? Did you use a flare wrench, to ensure all connections are tight? Did you check the connection where the new brake hose goes into the caliper--is the machined surface true, did you use new copper washers? Did you bleed the brakes in this order: RR, LF, LR, and RF?

Back in the late 90's I worked at a place and was ASE certified in brake systems and ABS systems as well. Funny thing though, the place I worked at had no ABS diagnostic equipment so I was kind of on my own.

A Ford Bronco came in for a brake problem...low pedal, very little brakes. Long story short...after replacing the master, bleeding 6 billion times, checking and double checking everything...there was a valve, similar to a proportioning valve located on the frame roughly under the drivers door in location. Out of desperation, I finally decided to replace that and re-bleed the system. That was it. I have no idea what the valve was called but I seem to remember it being referred to as a dump valve. Maybe it's something like this that you are running into.

Also...I have seen many people do this so it's worth checking....be sure the rear brakes are properly adjusted.

I recently worked on the brakes of a 1985 GM B6000 school bus for a Texas non-profit organization. It had a leaking hydro-boost, which was powered by the power steering pump. I had to install a remanufactured hydro-boost; reuse the vacuum pump, after replacing the O-rings; and repair a leak in the Saginaw steering gear. The point is, in spite of all the leaks in the booster system, I was able to change the brake fluid, bleed the brakes, and get a solid pedal. Therefore, I wouldn't focus on the booster system. Instead, I would get a shop manual and see if there is anything unique in bleeding the brakes and try to thoroughly understand the system. Some GM vehicles require a special tool (can't remember what it's called) that's used on the proportioning valve.