It is a 1979 Chevy truck long bed. I have no brakes. The brakes feel squishy/spongy. I filled up the brake fluid. We tried to bleed them, yet that did not work. I can pump the brake to build up pressure and it will stop. There was no puddle around my truck, and this happened over night. Could it be the Master cylinder? How can I check it if it is, and if I can.