I don’t know but that seems reasonable to me.

 

If it's a diesel, vacuum pump should be supplying adequate vacuum while driving.

If gas, as you remove your foot from gas pedal vacuum will be created to supply vacuum needed.

However, the brake booster should hold enough vacuum for two full brake applications...between usages (period of driving). Do you notice a hard pedal, after a lengthy period of driving (expressway driving)? Or, does engine run rough while pedal is applied at idle? There is a one-way check valve at booster end of hose from engine. It could be getting weak or rubber grommet could be failing. If you lose all vacuum within a short amount of time and those checkout okay, might be internal failure inside booster.

 

My brakes always feel the same regardless of driving conditions, but I've always felt that something might be wrong with the booster because the truck requires more pedal pressure than anything with power brakes I've driven before.

I'll definitely check out the check valve before I go back together.

The service manual doesn't give a vacuum test procedure. What's the max. vacuum I should apply for a test?

I'm really only messing with it because I'm about to replace the clutch master and slave so having to remove it anyway.

 

If your pedal feels somewhat hard, and check valve and grommet checkout good...replace booster. Sounds like you're loosing vacuum, while driving down the road...and getting very little assist.

Face it...it is over 25 years old...well, beyond any reasonable amount of time for failure to appear.