The quick-take-up piston is huge,
True.
if you've got those JB5/JB6 brakes with the QTU MC, and once the slave pistons have been "taken-up" the excess fluid needs to get shoved passed the cups of the "regular" MC pistons, at least for some duration of motion until the QTU piston is no longer in play, so there's that.
Not exactly.
The fluid from the bigass QTU chamber is getting pushed past the cups of the "regular" MC pistons UNTIL the caliper pistons have pushed the pads up to the rotor. Once the pads are against the rotor like a normal, "not low-drag" caliper, the fluid from the QTU chamber blows past a spring-loaded check-valve into the reservoir while the "regular" pistons/cups provide the higher PSI to clamp the pads to the rotor.
[One of many potential problems with chronic low-pedal issues on GMT400 trucks, is the need to balance the stiffness of that spring-loaded check valve against the force needed to move the caliper pistons. When the pistons get stiff (not quite seized, but harder to move than they should be) or if the check-valve spring gets weak or the check-valve leaks--all the QTU fluid is wasted by blowing into the reservoir, leaving the small "normal" piston to move enough fluid at higher pressure to get the pads up to the rotor.]
I don't know the RPO code. Regular ol' vacuum booster and 10" drums on the back.
JB/JN 3 or 5. JN3 had the weaker front brakes, smaller-bore master, and weakass power booster. Came only on regular cab trucks prior to '92, I think.
Most 1500 pickups had JB5.
Wild Guess: You need to bleed the ABS with a scan tool.
Second Guess: Power booster issues--not holding vacuum over time. Maybe as simple as a leaking check-valve in the grommet for the vacuum supply.
Third Guess: It's normal for a depleted power booster to have a hard, high pedal compared to a booster with enough vacuum to operate properly.