Well, if it was me, I would first make sure the fitting clamps are secure and tight. And be certain that all brakes were bled correctly. Are you losing fluid, but don't see any leaks? Is there any white smoke from the exhaust? These are signs of internal brake servo failure. If not, I would think a new master may be a next step. But please wait for some other comments to help guide you. A new brake master cylinder is never a bad idea on a car from the seventies, I mean we often do not know if it has ever been replaced. Good luck Haseeb and keep us posted.