There are adaptors to plug psi gauges into brake systems. I would suggest you find a loose gravel or dirt road. Going along at idle, slam on the brakes. You should skid to a stop. Don't move the truck. Get out and verify all 4 tires skidded.

If all 4 tires skidded, get a power booster so the brakes get more psi when at speed or start working out doing leg presses.

If just the front or rear skidded. Then you might have a bad part of the master to the other end, air in the line, or about 20 other things.

If 1 or 2 not on the same end skidded. Adjust your shoes and try again until all 4 lock up.