Try to buy from a recognized vendor with a reasonable return policy.

Option:
A master cylinder, if the inside bore is not badly pitted, is one of the easiest parts to rebuild. Classic Truck Parts has the kit for $24. Amazon has a hone for $10.58 (reusable on other projects).
Classic Trucks MC is $94.95 and has no core charge as an example. Therefore taking yours apart (with pics and notes) you can inspect, see how easy it is to replace parts and decide if you want a kit. If not, you have not lost anything and can throw it away. (SAVE ALL FITTINGS, CAPS AND RODS). You have to take it off the truck anyway. I assume your leak is due to internal rubber parts and not a simple fitting. If your cylinder bore is non pitted, just lightly run the hone in it, clean it all out with brake cleaner and begin rebuild. The "rebuild" is just shine the bore and reassembly.


Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn