This has been covered a dozen times or more. "Weak" brakes on C/K 1500s are epidemic, especially the first few years of production.
Your truck probably has JN3 brakes. You can verify by looking at the option-code decal in the glovebox. The CRAPPIEST power brakes ever installed on a GMT400 vehicle. They're so terrible, GM quit installing JN3 brakes after...'92, maybe. At that point, even the cheapest C/K trucks got JN/JB5 brakes.
1. The 254mm (10") rear drum brakes are garbage. They're always way out of adjustment, and they're just plain too damn small. The park brake has to be used, frequently, to get the rear brakes to adjust--and nobody uses the park brake. Then, the park brake cables seize so that they CAN'T be used. Result is low pedal, often squishy, and poor braking power.
2. The front calipers have the smallest pistons used on the GMT400 series vehicles.
3. The rotors are the smallest/narrowest used on the GMT400 series vehicles.
4. The master cylinder has the smallest primary and secondary piston diameters used on the GMT400 vehicles
5. The power booster is the smallest used on GMT400 vehicles.
JN/JB 5 or 6 booster compared to JN3 booster:
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6a. The front calipers and the master cylinder are "special" in that the calipers have a low-drag feature for improved fuel economy, and the master cylinder has a third chamber to supply a low-pressure gush of fluid to compensate for the low-drag calipers.
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6b. If the calipers have sticky pistons, or the calipers don't "float" on the lubricated rubber mounts/pins/sleeves, the low-pressure "gush" of fluid is blown into the reservoir instead of doing something useful, like move the piston. Brake pedal is low 'n' squishy, kinda like there's air in the system.
7. No one remembers to bleed the ABS unit.
SOMETIMES, the ABS unit is faulty.
A faulty ABS unit is the LAST thing I'd dick with, only AFTER verifying everything else. If one of the two valves is stuck closed, the pedal is high and firm, but with poor braking power because the rear brakes DON'T APPLY. If the other valve is leaking, the pedal is low and squishy as if there were air in the system, and the rear brakes don't apply much or at all.
Better brakes are REALLY EASY since the OEM JN3 brakes are so terrible.
The FIRST thing you should do is scrap the 254mm Leading/Trailing rear drum system, upgrade to the 11.x Duo-Servo drums. This means replacing EVERYTHING on the rear axle. When it was my
K1500 with JN3 brakes, I replaced the entire rear axle with one from a K2500. Stronger, bigger axle, stronger, bigger brakes. Problem is, that's more complex with a 2WD because the better rear drums also come with six-lug axle shafts, which means 6-lug wheels instead of 5-lug.
There are work-arounds, but it's not as cheap 'n' easy as it is with a 4WD. You're looking for a JN6/JB6 donor vehicle, and you'll be buying new drums, wheel cylinders, hold-down springs, etc. for a vehicle with JN6 brakes. But you'll have to get the backing plates from the Treasure Yard.
Upgrading the rear brakes was at least as much of an improvement as upgrading the front brakes AND upgrading the master/booster.
Next, you need to upgrade the front rotors and calipers. I think this is a bolt-on for 2WD--just buy the thicker rotors, and get calipers with the bigger pistons. Tell the parts counterman that you need parts for a vehicle with JN5 or JN6 brakes.
At the same time, you'll want the JN5 or JN6 master cylinder and power booster.
When it was me, I got everything from the Treasure Yard. I pulled the calipers apart for cleaning and inspection; and cleaned the master cylinder. On the 4WD, the front wheel bearings/hubs are different, but I don't think that's true with 2WD.
JN5 and JN6 are the same at the front, and the same at the master cylinder/booster. JN5 still uses the horrible 254mm rear drums, JN6 uses the much-better 11.x Duo-servo rear drums. Later vehicles use "JB5" or "JB6" instead of the JN numbers. Otherwise identical.
About all that's left is to THOROUGHLY bleed the system including the ABS unit; after assuring that the wheel bearings in front are properly adjusted and the brake hoses aren't plugged/restricted. And make sure the park brake cables aren't seized.
Basically, you're going to scrap EVERYTHING at the wheels, plus the master/booster.
IF (big IF) by some miracle you already have JN5 brakes, you've already got the good front brakes and master cylinder/booster. You'd just need to upgrade the rears,
and make sure everything is working properly including flushing the brake fluid. IF you have JN6 brakes, you've got good stuff already, just make sure it all works properly, isn't worn-out, and is flushed with fresh fluid.
