...he purchased his under-floor M.C. from one of the reputable parts suppliers,but it's designed for pick-ups,not the 1-1/2-2-tons. The bore is smaller...
It's reasonably important to have the right size bore MC for the brakes you're trying to use it for. A big truck has large bore wheel cylinders and needs a larger bore MC (than one from a small truck) to move the required fluid to operate them.
Good starting point is to replace the MC with the same bore size as the old one.
