1965 Valiant low brake pressure

Ok back to the problem that you lost the pedal after wheel cylinder replacement. When you changed them there are pins that go in the rubber cups and push against the shoes. Did you transfer them to the new cylinders? and make sure they are bottomed in the shoe because as I said earlier if they have travel to apply your pedal has travel. Again what people are failing to read here is he was ok till the cylinders were changed so most likely that is where the problem lies.