What's up guys,I got a 1973 chevy C10 cheyenne.I'm having brake problems but first I'll tell you the parts I've replaced with new ones. brake master cylinder,brake power booster, front brake calipers, rear brake cylinders, front rubber brake line and rear rubber brake line,front brake pads,rear brake shoes and proportioning valve. I bled out all the old brake fluid,bled the brakes in the right order.I adjusted the rear shoes to where they are barely rubbing the drums and I benched bled the master. so the problem I'm having is when I go to press on the brake pedal it'll be firm and the pedal will be at the top of its travel,then I'll come to the next stop and press the pedal down again and it goes to the floor,about a couple of inches from the floor board.if I step on the brakes real quick the pedal will go to the floor.sometimes the pedal will be good and then other times it'll go to the floor.I've bought 3 different master cylinders thinking that maybe it might be defective. this problem I'm having with my truck is really stressing me out,so if anybody has experience this kind of problem with their truck I'd appreciate your help very much.