I went to Larry at Prairie Grove Auto and he ordered a rebuild kit, Carquest part# brk M448, about $50. The chassis is a p375 and the MC piston diameter is 1.25".
I disassembled the MC and bead-blasted it, but I still need to hone it and paint the outside. One of the curses of the MC location in the Travcos is that it is under the floorboard in a dirty environment and hard to fill and even check the level. I have decided to design and install a remote reservoir for the master cylinder. Roughly copying best examples on the net, I have drawn a cover for the MC that will be tapped with two fittings, one over each OEM reservoir, made from 3/8" aluminium plate and bolted down onto a gasket. The fittings are connected to one hose each that will go up to a Mitsubishi Eclipse remote reservoir from one of my parts cars and mounted somewhere TBD, just above the MC to allow proper gravity flow. I have access to a CNC mill and will mill the cover early next week. I will also attach the low level switch in the Mitsubishi reservoir to an LED I will mount in the dash to warn of low fluid level, but even if it is empty, there should still be liquid in the OEM reservoir. I plan to use DOT4 fluid and hope to find speed bleeders to bleed the system often to mitigate water absorption. The MC failed as it had rust sludge and pitting in it, the result of moisture. BLEED YOUR SYSTEM OFTEN! (I need to follow my own advice BTW, LOL).
No pictures yet, sorry.
The '67 MC has a "flat" oval/rectangular cover that is held in place by a wire snap/clip. The '66 has a screw-on cap (cap has male threads) and I will be designing a remote unit for it after we complete repairs to the brakes on my brother's '67.