Is the '67 MC a disc brake MC? A disc brake MC won't have the residual pressure valve behind the line seat.
Make sure the rear piston in the MC is fully returning when the pedal is released. If pushrod adjustment is preventing the piston from fully returning it will trap pressure in the system when the pedal is released.
^^^This makes sense..you should be able to take the cover off the MC, then have some watch while you depress the brake pedal hard. Then release quickly and you should see backflow in the fluid if the hole is uncovered.
DON'T forget to throw a fender cover on.
