Who's fought Silverado/ Sierra brake problems? and won?

I'm so stupid. When I tried out this (otherwise very nice low miles) 04 Sierra, it seemed OK. But this was with the OWNER on board. I was reluctant to really "ring this thing out."

First stop in the morning you can nurse the pedal down and press hard, it goes to the "stop." (Bottom of stroke whatever it's hitting, bottom of master?) The pedal, that is, feels somewhat spongy, air?

It stops "normally to moderately" but has "no more" In fact it won't even activate the ABS until fairly slow speed

A panic or loaded situation would be unacceptable. Previous owner not cooperating, claims Perfection Tire declared it "fixed." I even paid a local shop, previous GM mech who told me "these are excellent" Bullshit!!!!

Who has fought similar problems with these? It's gas, has an electric vacuum boost pump hung onto the booster. I don't have the service manual "yet" has 4w disk brakes.

So? poor boost? Air? both? ABS problem?

The internet has so many hits it's actually of little help