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Old 06-08-2012, 02:45 PM
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GM drum brakes not working

What are the logical things to check for drum brakes not working. This is for an 84 C10 with hydraulic rear drum brakes. I jacked up the rear end, spun the wheels by hand, mashed the brakes, and the tires continued to spin without resistance. The master cylinder has fluid and the front disk brakes work fine.

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Old 06-08-2012, 03:43 PM
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Quote:
Originally Posted by Silver Surfer
What are the logical things to check for drum brakes not working. This is for an 84 C10 with hydraulic rear drum brakes. I jacked up the rear end, spun the wheels by hand, mashed the brakes, and the tires continued to spin without resistance. The master cylinder has fluid and the front disk brakes work fine.
I would think the proportioning valve would be the first thing I would check.

See if you can bleed the brakes in the rear.

Brian
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Old 06-08-2012, 07:12 PM
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Most likely the wheel cylinders are rusted frozen and need replacing.
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Old 06-08-2012, 07:13 PM
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The combo valve (most people call it the proportioning valve) combines a differential pressure switch in addition to the prop valve (hence the term COMBINATION valve). If the brakes have been leaking or bled recently, the diff pressure switch can get stuck all the way to one side. This blocks fluid flow to that side, since the diff pressure switch "thinks" there's a leak and is closing it off. If the piston in the diff pressure switch is stuck due to rust in the combo valve housing, the only fix is to disassemble it or replace it.
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Old 06-09-2012, 11:47 AM
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Quote:
Originally Posted by Cape Cod Bob
Most likely the wheel cylinders are rusted frozen and need replacing.
You know what, that HAS happened to me. It was only one side but who knows, it could be both. I think that is exactly where he should start is by pulling off the drums and getting a good look at that stuff. If the drum hasn't been pulled or and inspected you know nothing. You are so right, I moved forward too fast, check those wheel cylinders!

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Old 06-09-2012, 07:59 PM
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Quote:
Originally Posted by MARTINSR
You know what, that HAS happened to me. It was only one side but who knows, it could be both. I think that is exactly where he should start is by pulling off the drums and getting a good look at that stuff. If the drum hasn't been pulled or and inspected you know nothing. You are so right, I moved forward too fast, check those wheel cylinders!

Brian
A quick way to check is to open the bleed valve(if it isn't frozen)(crack the line to the wheel cylinder if it is) and depress the pedal to see if any fluid is coming thru and take it from there
Hope he lets us know
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Old 06-10-2012, 08:43 AM
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I just had to replace the rear cylinders on my dump van last week. Both were frozen. Saying that, 6 months ago I had to replace the front calipers. Same reason. This vehicle moves less than 50 miles per year. Point being if the rears are frozen you might want to figure a whole brake job. If the rears are junked up so is the rest of the system. I also installed a new m/c with the rears. you have to keep in mind that these are your brakes and you might not want to talk to the guy you just rear-ended.
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Old 06-10-2012, 09:19 AM
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Quote:
Originally Posted by Cape Cod Bob
A quick way to check is to open the bleed valve(if it isn't frozen)(crack the line to the wheel cylinder if it is) and depress the pedal to see if any fluid is coming thru and take it from there
Hope he lets us know
this is the best starting point.
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Old 06-11-2012, 09:16 AM
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Thanks for all the replies! I will start with a bleed test. My initial assumption is that the wheel cylinders are frozen. This is an old truck that sat around for quite a while before I bought it.

Bleed test:
Get fluid-> wheel cylinders frozen
No fluid -> proportioning valve
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Old 06-11-2012, 09:58 AM
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You could have a plugged master cylinder and not get fluid flow. Crack the fitting at the m/c to check this.

If your system has frozen components you really ought to go through the whole system. Have you checked out towing charges lately? I just paid $275 out of pocket and the insurance co. paid a additional $150 to have one of my vehicle towed Put your money against your brakes. Everything clean and new.
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Old 06-11-2012, 12:09 PM
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Quote:
Originally Posted by boothboy
You could have a plugged master cylinder and not get fluid flow. Crack the fitting at the m/c to check this.

If your system has frozen components you really ought to go through the whole system. Have you checked out towing charges lately? I just paid $275 out of pocket and the insurance co. paid a additional $150 to have one of my vehicle towed Put your money against your brakes. Everything clean and new.
Yes I agree with that. I am strongly considering taking the truck to a shop and having them do a top to bottom brake job. I just don't have the time lately. I can't imagine what they would charge though...tow truck is probably cheaper LOL!
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Old 06-11-2012, 12:19 PM
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Quote:
Originally Posted by boothboy
You could have a plugged master cylinder and not get fluid flow. Crack the fitting at the m/c to check this.

If your system has frozen components you really ought to go through the whole system. Have you checked out towing charges lately? I just paid $275 out of pocket and the insurance co. paid a additional $150 to have one of my vehicle towed Put your money against your brakes. Everything clean and new.
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Old 06-12-2012, 10:15 AM
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What kills me is you guys are right and I need to go thru the brakes in my Gran Sport before I drive it. It's been parked for 12 years and there is likely something wrong. DAMN, another thing to do!

Brian
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