Im working on a 96 gmc sonoma 4x4. I had to replace rear wheel cylinders and rear brake lines since they were rusted out. I bleed the brakes and the brake pedeal is nice and firm. But when I start it the brake pedal loses all prusure and the brakes hardly work. As soon as I turn the truck off and pump them I get hard firm brake pedal again. Abs light has been on for a long time im told and is still on. Also when the truck is running and the brake is applyed the brake light comes on. Any ideas where to look next to fix it?