Brake pedal intermittently goes to floor unless I tap first then I get normal resistance

You should tear apart your front and rear brakes and inspect them like Lastchance suggested. If you have rear drums, they shouldn't be out of adjustment, because they automatically adjust on their own. I would first take apart the front discs and make sure that the slides aren't rusted up. I would then take off the calipers and depress the piston with a large C-clamp. If you can't compress the piston, then you have a seized piston. If it compresses fully, have someone slowly push on the brake pedal to see if the brake piston moves outwards. If all functions properly, reassemble your front brakes. Now disassemble the rear drums. You will have to remove the rubber plug in the rear of the hub and loosen the star wheel. When you get the cover off, have someone slowly push on the brake pedal and see if your pistons are moving the shoes. You could have a seized rear piston. If pistons are moving properly, reassemble your rear brakes. If all brakes are functioning properly,with no leaky pistons, I would then try to bleed the system. If no air, I would then spend the money on a new master.