Hi the brake valve is a common cause of a sinking pedal on your model truck. I have replaced many of the valves for this problem. Here is an article that describes the problem and how the manufacturer recommends it be testedI have and alternate method that I leared at a brake clic some years ago. It requires a spare end cap, to use for testing,it has a small 1/8" hole drilled in it. I remove the end plug on the truck and install the test cap.I insert a rod in the hole until let it bottoms out,then apply the brakes,if the rod moves,it indicates the valve is bypassing internally and needs to be replaced.
Kelsey-Hayes rear wheel antilock (RWAL) systems have been gotten a bum rap in recent years. The RWAL system on 1991-96 Chevy S-10 and S-15 pickups, Chevy Blazer, GMC Jimmy, 1992-95 Chevy/GMC Suburban and 1990-95 Chevy Astros have been the subject of numerous "investigative reports" by various news media, the target of a class action lawsuit (dismissed in August '87) and an NHTSA investigation which found nothing (in spite of having received more than 7,000 complaints including 1,600 accidents and more than 500 injuries!).
The Kelsey-Hayes rear wheel antilock brake system was first used by Ford in 1987 on F series trucks, and was later added to the Ford Ranger, Bronco, Bronco II, Explorer, Aerostar and Econoline vans. Ford calls their version of Kelsey-Hayes EBC2 system "RABS" for Rear-wheel Antilock Brake System."
The GM RWAL version is found on '88 and later "C" and "K" full-size pickups, "S" and "T" series pickups, some "S" series Blazers, '89 and up Astro minivans, '90 to '92 "R" and "V" series light trucks and "G" series vans.
Dodge has also used the Kelsey-Hayes RWAL system since '89 on its "D" and "W" 150/350, Dakota and Ram pickups. Geo, Isuzu, Mazda, Nissan and Subaru have used the system since 1991.
Considering how many Ford, Chevy, GMC and Dodge trucks have the Kelsey-Hayes rear wheel antilock brake system, it's not surprising that a certain number of these vehicles would experience some kind of problem during their lifetime. NHTSA says there's no inherent defect in the system, so any failures that occur are the result of normal service conditions.
One of the most unnerving failures that can occur with this system is the loss of pedal when braking. The problem may feel like a bad master cylinder, but it may not be the master cylinder. The real problem may be a bad Electro Hydraulic (EH) valve in the rear wheel antilock brake system.
If a small piece of dirt or rust gets into the unit, it may prevent the dump valve inside the EH valve from closing. The dump valve will then leak fluid into the accumulator when the brakes are applied. The misrouted fluid allows the pedal to drop, and the pedal may go all the way to the floor without applying the brakes. No ABS warning light or fault code will be found either because the limited diagnostics on this system can't tell if the dump valve is fully closed or not.
Wagner Brakes recommends the following procedure to find out if the problem is a bad master cylinder or a leaky EH valve in the RWAL system. To rule out a bad master cylinder, disconnect the rear brake line at the master cylinder and install a plug to block off the port. Have an assistant depress the brake pedal about an inch to purge any air from the outlet, then tighten the plug while the assistant holds the pedal in this position. After tightening the plug, continue to apply pressure to the pedal to prevent damage to the primary cup in the master cylinder as the cup moved across the vent port.
If the pedal holds and no longer drops, it isn't the master cylinder that's causing the problem.
Reconnect the brake line to the master cylinder and block off the outlet port on the EH valve. Then apply pressure to the brake pedal again to see if the pedal drops. If the pedal goes down, the EH valve is leaking fluid into the accumulator. The EH valve needs to be replaced.
It's important to note that one of the underlying causes of EH valve failure is moisture-contaminated brake fluid. If the fluid is more than a couple years old, it may contain a lot of moisture. GM trucks do not have a recommended service interval for brake fluid, but most experts say that flushing the brake lines every couple of years for preventative maintenance can greatly reduce the risk of internal corrosion inside the brake lines and EH valve. But most people totally ignore their brakes until something goes wrong. So the best you can do is flush the system and replace the old fluid with new when the EH valve is replaced -- then recommend a fluid change every two years to prevent a repeat occurrence.
Air trapped in the EH valve can be another source of trouble with the RWAL system. Some RWAL EH valves (the smaller ones) do not have a bleeder screw, which makes the unit hard to bleed if air gets into the lines. The trick here is to loosen the brake line connections at the valve to vent air when bleeding the brakes.
Brakes can be bled in the usual way manually or with pressure equipment. The wheel bleeding sequence is RR, LR, RF, LF. If a pressure bleeder is used, the combination valve must be held open.