If you order a replacement master cylinder and it comes with a rubber boot similar to the one shown in the link below, don't use it. It will prevent the piston from fully retracting and your brakes will lock up. If the piston does not fully retract, the return bleed hole will be blocked.
https://www.larrystbird.com/product/mas ... boot-5860/
1965/66 Master cylinde r
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