It looks like I need a new master cylinder for my 1966 F100. Advanced Auto Parts and AutoZone have some exact fits for $24, and then some for $130.

It is just an old truck, that I drive to the dump on the weekend. Does it really matter which on I choose. Is there somewhere else I should look at to buy one?

 

Yes. I suggest you stay away from remanufactured mc's. A new one should be available for well under $100.
Eric

 

I bought the cheap one Fenco Reman/Master Cylinder (Brake System) (M37781) | 1966 Ford F100 1/2 ton P/U 2WD 8 Cylinders 5.8L 2BL OHV | AutoZone.com . I don't see any difference and you save a lot. The only trouble I had with it was finding the fitting. Wilwood 220-0628 Wilwood Fittings & Adapters - Free Shipping on All Orders @ JEGS For me I couldn't hand drive the fitting into the master cylinder so I suggest putting the master cylinder upright in a vice. Once you have it secured start the thread on on the fitting if its anything like mine you'll only be able to thread a small amount. I'm talking like 1 thread, but just getting it started is key. Use a socket wrench to slowly drive the fitting further and further until its bottomed out.

If the fitting doesn't seem to fit try again, it fits.