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06-05-2002, 01:18 PM | #1 |
Registered User
Join Date: Jun 2002
Location: Camas, Wa
Posts: 58
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71 gmc 2500
Just purchased a 1971 gmc 2500 4x4 and I have a few questions:
1 rear wheels lock up when put on the brakes, I changed cylinders (shoes looked fine) what next? I also am trying to get brakes shoes and I am told that it takes 12" brakes, however I got 12" and they are too small, maybe this is a one ton rear end? |
06-05-2002, 01:30 PM | #2 |
Recovering Truck Driver
Join Date: Mar 2002
Location: Orleans, NE USA
Posts: 1,883
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hmmmmm.....does the 3/4 GMC have disc on the front???.....sounds like maybe the proportioning valve isn't doing it's job and applying too much pressure to the rear....also check the return springs, they may be getting weak.
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67 K-20 350, SM465, Eaton rear, 4.56 no spin option 00 Dodge 2500 4x4, 24V cummins, 5 speed Chad South Central Nebraska |
06-05-2002, 01:34 PM | #3 |
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Join Date: May 2002
Location: Oakley, Ca. USA
Posts: 137
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Either the proportioning valve isn't working, of the fron calipers need to be rebuilt (they aren't supplying the stopping force they are supposed to, thus causing the rear to take up the load). This was occurring on my '71 1/2 ton.... After the front calipers were rebuilt, had no further problems.....
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Rudy Fierros '95 Honda VFR750F Used to have a '71 Chevy Stepside, Guards Red, Lowered 4" rear, 3" front, White Oak bed wood, 700R4 w/373 gears until some low life stole it! |
06-05-2002, 01:34 PM | #4 |
Weapons Of Construction
Join Date: Apr 2000
Location: Colorado, USA
Posts: 1,095
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Try bleeding the brakes too. You may have air on one side.
Also, check for an out of round drum. Have someone push the brakes gently while you watch the shoes move with the drums off. Maybe one of them is bound up or something.
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1971 GMC 2500, 402/TH400 4.10 Daily Driver Lafayette, CO |
06-05-2002, 02:18 PM | #5 |
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Join Date: Apr 2001
Location: galesburg, illinois US
Posts: 40
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Have same problem
i had to finally back off the rear adjusters so they would not lock up except under really hard braking. i was told that it was my proportining valve. and as far as the back brake shoes. i have a 71' 3/4 ton 2500 and it has the heavy duty rear end. there were two sizes of shoes depending on the rear end. ones are 12" by 2 inch and the others are 12x2.5 i think. i had to have the larger ones for my truck.
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06-05-2002, 02:25 PM | #6 |
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Join Date: Jun 2002
Location: Camas, Wa
Posts: 58
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yes it does have disc on the front.
the rear cylinders were leaking pretty good before i rebuilt them. can brake fluid contaminate the linings? the pedal also seems a little hard. I might try turning the drums first then put some new shoes on and then go towards the front from there |
06-05-2002, 03:22 PM | #7 |
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Join Date: Jul 2000
Location: Victoria,BC, Canada
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One thing you could try is resetting the proportioning valve by depressing the little button on the front of the prop. valve while you pump( or bleed?) the brakes.
I remember reading this but I have not tried it, I recall there was a special tool you could fasten to the body of the valve which would hold the button down while you pumped. I can't recall the details but its in the service manual.
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71 GMC 1500 LWB |
06-05-2002, 03:49 PM | #8 |
CCRider
Join Date: Feb 2000
Location: Olive Branch,MS,USA
Posts: 2,232
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Brake fluid or oil from a leaking axle seal will make them lockup. That is always the first thing I look for when rear brakes lockup. If the brakes shoes are contaminated, I would try sandpaper on them first, it will work if they are not too bad. If they are soaked all the way thru you will need new shoes.
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72 GMC Sierra SWB almost finished---- 84 Softail Olive Branch MS |
06-05-2002, 10:30 PM | #9 |
its all about the +6 inches
Join Date: Jan 2001
Location: Hilliard Ohio
Posts: 2,690
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Having your brake shoes soaked in fluid can also cause a fire. (very extreem situation) but I have winessed it myself in Cail when a truck lost controll coming down a long hill.
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06-05-2002, 10:51 PM | #10 |
"See the U.S.A....."
Join Date: Apr 2002
Location: Richmond TX.
Posts: 269
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definately change the shoes.
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1970 short bed fleetside 1949 all origional chevy 3600 1974 camaro Richmond , Texas(just S.W.of Houston) |
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