1979 GMC k1500 4x4 brake problem

[AZ-truck-man]

I have a K1500 1/2 ton 4x4 Sierra Classic with a 350 and the brakes do not have pressure. I have replaced the vacuum booster, master cylinder, front rotors and calipers, rear shoes, etc. Basically everything is new except for the combination valve. However, when I pump the brakes they build up pressure, I wait about 10 seconds and it loses all pressure and goes to the floor. I have gone through 3 master cylinder, what could the problem be?

[00Silverado4x4]

Bleed, bleed, then bleed some more. If you've replaced that many items in your brake system you have lots of air to bleed off. After that, I'm not sure unless you have a leak somewhere.

[AZ-truck-man]

I have bled and bled at least 50 times, air keeps finding its way into the system, mainly in the rear brakes, something is creating air in the system. Could it be the proportioning valve?

[00Silverado4x4]

Could be. I have an old '76 and mine was a wheel cylinder blew up. Got it replaced just for the rear brake line along the frame rail above rear axle blew. If you think it's in the rear, check out from the wheel cylinders forward to the proportioning valve. I'm willing to bet you've sprung a leak somewhere. Our trucks aren't exactly young chickadee's anymore. lol

[gmtech4]

did you bench bleed the master cyl?

[txab]

did you bench bleed the master cyl?

 

My first question as well....

[AZ-truck-man]

Yes the master cylinder has been bench bled

[AZ-truck-man]

It was the brake proportioning valve, it had a hole where it was soldered and must have been sucking in air by the bleeder valve, thanks for the suggestions guys

[00Silverado4x4]

Glad you found the problem.