Hi, I have a 95 Chevy Blazer that had a brake line going to the back of the truck rust out. In the process, the brake warning light cameon. I replace the brake line running to back of the truck, the distribution block at the rear axle, and the lines running to the wheel cylinders. I ended up replacing the rear wheel cylinders because the bleeders were rusted in. I bleed the rear brakes. No air appears to be coming out of the bleeders any more. The brake pedal is firm but lower than before, but the brake warning light is still on and I have no rear brake pressure. Is it possible that the combination valve is stuck or frozen? Could I still have air in the rear brake system?