Yukon brake problem

Hello to all,

I have a 1997 5.7L Yukon & I changed the front brakes on my Yukon, used a C-clap to press front brake piston in w/o having to open the brake line. Once completed w/ this process turned on the Yukon & pumped the brakes as I have done many times in the past. The brakes are very spongy w/ the Yukon engine on (brake pedal flush to floor board). Bled the brakes RR, LR, FR, FL using a vacuum gun & having someone pump & hold brakes. Brakes are hard as a rock when Yukon is off, but once the motor is turned on, the brakes are spongy all the way to the floor board. Replaced master cylinder, no luck, had GMC dealership flush brake system, still no luck. I am running out of ideas...PLEASE HELP.:rofl: :rofl: :rofl: :banghead:

Well I have an 96 tahoe....did u try to bleed the brakes in the same sequence but let the fluid run on the ground? and did u also do this sequence...pump, pump,pump,pump...hold...unscrew...tighten. That normally work..and also what a lot of people dont know..u also need to adjust the back shoes so all 4 brakes hit at the same time!!!! that will get a lot of play out the brake pedal!!!