I have a 72 dart started leaking from the master cylinder where it meets the brake booster is there an o ring i can purchase to stop the leak or woul di have to buy a new master cylinder??

You need a new master cylinder (or you need to rebuild the one you have). New ones are still available; don't buy a "remanufactured" one. Does your Dart have disc or drum front brakes?

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

it has disc in the front drum rear with brake booster