GMC Troubleshooting Questions? Ask a Mechanic for Answers
What method of bleeding were you using, a vacuum pump, a pressure bleeder or the foot on the brake pedal (manual) method?
Ok, it's been from my experience that using a vacuum pump cannot be done when there is a line replaced or something like that. It's just too much air I suppose. I brake pedal method is the best. Do you have someone else pumping the brake pedal? Here is a method I found works the best, ***** ***** the person pumping the brake pedal cannot make a mistake when you are yelling out, down, hold, up or whatever. Just loosen the bleeder and then hold your finger or thumb over the hole and have them slowly press the brake pedal up and down. Quickly might cause bubbles.
Or if you are by yourself, the ole method of attaching a hose to the bleeder and setting the end of the hose in a container or jar with brake fluid already in it so when you let up on the pedal it won't suck in air but brake fluid.
Other than that, if you still cannot bleed, check for a leak.