1965 Valiant low brake pressure

Yes a internal leak in the MC will almost always result in a pedal that gets lower as you hold it.
Again I suggest next checking your pedal free play. I dont know how '65 are but on my '70 there is a bolt you loosen on the bracket for the brake switch to move it back and forth for adjustment. Loosen it and allow the pedal to come all the way back and then test for pressure then adjust the bracket to the new pedal height.
I think your WC are fine(unless they are Autozone 'brakeware' brand)