Next test please try this
When it locks try to open the bleed screw at the locked wheel....if it spits you have either a blockage in the master,wheel cyl or line
If you were really boiling the fluid the brakes would go to the floor !First time! You'd have to pump them
I've seen masters with the wrong plunger that blocked 'bleed back' and retained pressure.
If the brakes are adjusted and bleed correctly it should lock everything the same rate
One more thing ...Jack the truck up and see if the other rear wheel stops when you step on the brake
Try this when all wheels are cold and free
If you feel a difference in the brake action left or right that will give us a clue
Is there a return spring on the brake pedal?
Is the pedal binding in the floor board?
