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09-21-2009, 08:05 AM #1
Bleeding brakes on 1977 GMC, problems!
Ok, we are attempting to bleed the brakes on my 1977 GMC pickup (its actually the 1955 GMC pickup on the 77 frame, with the entire braking system and frame from the 77 truck. Anyways, my father is a mechanic, and we are running out of ideas.
We started having issues with the braking late last year, as the passenger side front wheel would lock up with minimal brake pedal pressure. Dad's thought was that it was the brake proportioning (combination?) valve. We took it apart, and found nothing but a broken seal inside of it. After trying every auto parts store thinkable, we went to a pull-a-part, and pulled at valve from a comparable truck. We attempted to bleed the brakes, and got trace levels of pressure at the bleeders. We took the master cylinder back to autozone, and did a warranty swap, replaced it, and have the same problems. Still no pressure to the bleeders. We "bled" the system at the proportioner valve and had good pressure into the valve and out of the valve.
I have read that the flex hoses can go bad, but would one hose being bad cause the pressure to be lost to all wheels, or would all 4 go bad at once? This is our last "idea."
Help, Please! I am in the process of buying a new home, and this truck needs to be sold to cover part of the down payment, so we are in somewhat of a hurry.
Thanks guys, for all your help on this matter.
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09-21-2009, 08:46 AM #2
Take a look at the diagram. As you can see, after the proportioning valve the lines are separate to each front wheel and common to the rear tee before separating to the individual drums. It's hard to believe that all four lines are blocked.
You need to isolate the problem so start with an air compressor. Pressurize the brake lines after the proportioning valve. Open all the bleeder valves and check for air flow at each corner.
Either the lines are blocked or the proportioning valve/master cylinder is not working correctly. If the lines are clear, take another look at that proportioning valve/master cylinder. If any of the lines are blocked replace them.
Since the only common point is the master cylinder or proportioning vavle I would suspect one of those as the culprit.
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09-21-2009, 09:33 AM #3
My vote is that there is something wrong with the proportioning valve. If you can't find a stock replacement, you can always look into a basic aftermarket one.2010 Equinox LT
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09-21-2009, 09:39 AM #4
We did replace the stock valve, with a valve from another truck, with the same results. Even if the valve is bad, wouldn't I eventually get some pedal pressure after pumping for 3-5 minutes?
09-21-2009, 10:43 AM #5
possibly, unless you pumped your reservoir empty, due to a bad o ring or gasket or something. Plus, if there is something wrong inside the proportioning valve, if is possible that it would just flow back into the master cylinder.
09-21-2009, 01:53 PM #6
Have you used a bench bleeding kit on the master cylinder yet?
One of the easiest ways to bleed brakes is to let gravity do it. Takes time, but it's effective. Fill the reservoir, and follow the line along, cracking the first fitting. When you have nice consistent drips, go to the next one. Finally, the bleed screw. If there are loops in the line, you can tap them with a tool to speed the bubbles along.
Another thing you might try at this point, is to use some self bleeding bleeder screws. Get a couple of quarts of cheap fluid, and just go to town. And remember, only one bleed screw open at a time.
Repeatedly depressing the brake pedal just re-compresses the air in the line each time.When you hear hoofbeats, look for horses not zebras.
09-21-2009, 02:03 PM #7
We have bench bled the master cylinder. I guess we will try the gravity deal. Just kind of stumping us as we have good flow through the proportioner and nothing at the wheels...
09-21-2009, 10:12 PM #8
Are you sucking air at a loose fitting? Maybe you have a bad flare? (Or forgot to flare? OOOPS!)
09-21-2009, 10:13 PM #9
definitely try to gravity bleed, that just might be the entire problem...
09-22-2009, 08:15 AM #10
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