My trucks sit for months without moving and I never have a pedal issue. Sounds to me like the MC needs to be rebuilt.
Putting a 1/2 ton MC on a 2 ton truck may work, but you rick not having enough pedal travel to stop the truck. The small diameter MC can't push enough fluid into all of the wheel cylinders, that's why GM made larger MC's for big trucks. Why not find a larger diameter modern MC so you have lots of pedal travel?
Mike B
