I'd stick with what you have, it is hard enough to find a parts store that knows what a medium duty truck is, let alone how to find parts for one that hat been modified.. I found a place named Reman Inc that will have anything you need to rebuild or buy a rebuilt HydroVac. Google Reman Inc for contact info. If you insist on going with a later brake master cylinder setup, be sure you get acquainted with brake pedal ratios, wheel cylinder diameters and master cylinder diameters. There's a lot more to this than bolting on parts. There is an excellent reference article on Pirate 4x4.com, written by Billavista. Good luck.
