thinking about buying 96 4.0 se

I'm considering buying a 96 4.0 se with 107,000 miles. The seller told me there are some problems with the brakes. He said he put on new discs, new rotors, and had the system bled, however the brakes still are not working properly. The brake specialist told him it might need a new master cylinder. How much would this cost? Would I be making a mistake buying this truck?