Just how far does the pedal move when fully standing on the brakes? How much clearance from pedal to floor at full travel ( standing on pedal )?
Have you really tried a panic stop in a parking lot? 35 mph stand on brakes panic stop? 40 mph? 50 mph? Everyone should to know how the truck is going to act!
What is the surface of the rotors like, polished smoothed, slightly polished, dull, grooved?
How about the pads, worn some, worn none, real dusty, slightly dusty?
How many stops can you make in a short span before running out of vacuum? Or does it always have plenty of vacuum on the booster?
Are you 100% sure the master cylinder is hooked up ( plumbed ) correctly? front and rear in correct locations?
Pedal travel is determined by master cylinder bore, brake pedal ratio, shoe clearance, and pad clearance, that's why I ask.
Panic stop, we need to know what it actually does?
Rotor surface is controlled by pad material, harder pads polish, softer pads wear out and make dust.
Vacuum going to booster can effect how well the booster performs, low vacuum = low assist.
Master cylinder plumbing, causes wield braking issues, they stop, but not very well. I have done this.
Any pickup should be able to lock the rear brakes fairly easy, even stock drum brakes will. Yours should no matter what is on the front axle. Master cylinder pressure should be checked before changing bores to see what you have now. I bought a 5000psi gauge for this, but didn't need it once I saw the error of my plumbing mistake at the master cylinder.
2 psi residual valves take up pedal travel, they do not improve braking performance. If you quickly pump the pedal, does it get stiffer and stop better? Does it always build pressure at the same pedal travel distance?
If the travel distance changes with a quick pump or two, then residual valves will help, but they won't change how the truck stop. They will only improve performance if the master cylinder is towards the end of it stroke before full pressure is met, but the quick pedal pump will show this. You would also have a very long brake pedal travel if this was the case.
I run a low mounted master cylinder with manual disk brakes with out residual valves and can stop just fine, every time at any speeds. I had to install an adjustable proportioning valve to limit rear pressure just so the rears didn't lock up under hard stopping.
