Is brake pedal effort high? Maybe th booster isnt your problem.
First thing to do is lift the truck, apply the brakes, and make sure both systems are working. If the combination valve is shuttled, one system wont be functioning. If the front system is shut off, you lose 60% of your braking system power.
Inferior brake pads can cause a low friction co efficient, causing a very diminished stopping power, even with ample pedal application pressure and assist from booster.
If you uhook the booster ( be sure to block vacuum hose so motor doesnt stall) and the pedal is much harder to push to obtain the same stopping distance from a set speed then the booster is obviously multiplying the effort applied to the pedal.A booster can hold vacuum and leak when brake is applied resulting in loss of assist, They can also hold vacuum and not apply any multiplication of force.
If you are pretty much applying brakes as hard as you can, and the truck takes forever to stop, it could be the pads are inferior.
If the booster is bad, you should still be able to stand on the brakes and make the wheels to skid. You might try that on a dirt road and have a friend watch to see that all 4 can be locked up.That is a good way to determine that all 4 are working.
Fact is stranger than Fiction