GM eliminated the residual check valves from the drum-brake master cylinder in the early '70s.
If they ever returned to having a residual check valve, I don't know about it.
The O-N-L-Y purpose of a residual check valve is to maintain slight pressure on the wheel cylinder seals, so that they don't leak air into the system. They're better at sealing fluid IN than sealing air OUT.
Four common reasons for poor rear brake performance:
1. AIR IN THE SYSTEM!!!!!!!! Air in the master cylinder, air in the ABS, air at the wheel cylinders, air ANYWHERE in the hydraulic system.
2. The rear brakes are the horrible leading-trailing shoe design, and they're out of adjustment
3. Rear brake hose (or metal tubing, or seized/blocked pressure valving--proportioning valve, holdoff valve, etc.) blocks fluid flow. I've had multiple brake hoses swell shut--tighter than a bull's ass in fly season. No fluid gets through. If you bleed the brakes and get reasonable fluid quantity at the bleeder screw, this isn't the problem.
4. Someone tightened the park brake cable, when they should have adjusted the service brakes.
There are other causes, these are merely four common ones.
For the record, if you've got "Low Drag" front calipers, installing a non-Quick Take-Up master cylinder will destroy brake performance, and create a low pedal in the process.
Pretty-much all 1/2-tons, and maybe the six-lug 3/4 tons have low-drag calipers. The original master cylinder will have a large-diameter piston (36 or 40mm) at the rear, next to the booster; and a "step" in the casting to accommodate it.
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