It might be possible to replace the master cylinder cups while the master is still in the truck, but I think that'd be asking for trouble. More than likely if the master needs cups, it also needs cleaned out. The main cup is 1 3/4, the backup cup is 1 3/4 O.D, 1 5/16 I.D.
There's new master cylinders on eBay for a little over a hunnert bucks, shipping and all. I've dealt with the guy and he is straight up.