06 Sierra Brake pressure problem

[Kahlebm]

PLEASE HELP!!! I have a 2006 GMC Sierra that I bought and it needed front pads and rotor due to the rotors being warped. So I did that as well as changed the shoes and hardware in the rear drums as well. Mind you, the brakes were ****** and barely there when I bought it. Even after changing everything out I have a spongy brake petal and the rear brakes won’t bleed right. I get a little air then some good fluid, then NOTHING. Like I’m not getting pressure. Front calipers bleed fine. I did notice that when you pump the brakes it sounds like the brake booster is making a hissing noise… please help. No visible leaks in the lines so I think those are fine. Just don’t know what to try replacing next..

[richard wysong]

sounds like a bad master cylinder