Power brake issues

[Jimal]

When I got home from work today I threw the charger on the battery and after letting it simmer for a little while I fired the truck up on the old gas that was in the new tank. It runs... sort of. I need to rebuild the carburetor and throw a tune up at it, but it has good oil pressure and apart from whatever was cooking off the exhaust manifolds, it doesn't smoke.

My unexpected problem is the brakes. The P.O. upgraded to dual circuit brakes and a power booster. Because we winched the truck on and off the trailer, we didn't try the brakes. When I went to start the truck I stepped on what felt like a very stiff brake pedal. After I started the truck, I realized that the pedal wasn't just stiff, it was stuck. I gave it a good, hard push and the it pedal went POP and went to the floor. Now the pedal goes to the floor with only the return spring for resistance. I thought maybe I popped a hose, but I'm finding any leaks and the master cylinder is full. I popped the check valve out of the brake booster and had my wife pump the pedal, and it pushed air. Any thoughts about what might have been stuck before popping?