So I'm sitting at a light about a mile from my house after a 4 hour trip home in my 1997 Chevy C1500. Just as I'm about to let off the brake, I feel the pedal sink. Thinking i have lost brakes, I pull into a slide road. I can push the pedal to the floor but can still stop the truck, but something definitely was wrong. Half way back home the brake warning light comes on. So today, checked all over for leaks, nothing. Even checked the back of the master cylinder, but still no leaks to see. Pumped the pedal a few times, but the brake warning light doesnt come back on. I can hear/feel rear brakes tightening up but not the fronts. I noticed then, that when I push the pedal down, brake fluid comes up into the larger reservior, so am guessing a seal has gone in the MC and the fluid is going back into the reservior and not to the wheel cylinders. Anyone suggest any other tests I can do?