71 gmc 2500 - The 1947 - Present Chevrolet & GMC Truck Message Board Network

kpf · 06-05-2002, 01:18 PM

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?

O'l Buck · 06-05-2002, 01:30 PM

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.

vfrdude · 06-05-2002, 01:34 PM

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.....

71GMC_3/4T · 06-05-2002, 01:34 PM

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.

ron68 · 06-05-2002, 02:18 PM

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.

kpf · 06-05-2002, 02:25 PM

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

Spoof · 06-05-2002, 03:22 PM

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.

tom hand · 06-05-2002, 03:49 PM

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.

Longhorn Man · 06-05-2002, 10:30 PM

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.

70shortnphat · 06-05-2002, 10:51 PM

definately change the shoes.

06-05-2002, 01:18 PM	#1
kpf Registered User Join Date: Jun 2002 Location: Camas, Wa Posts: 58	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
O'l Buck Recovering Truck Driver Join Date: Mar 2002 Location: Orleans, NE USA Posts: 1,883	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. __________________ 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
vfrdude Registered User Join Date: May 2002 Location: Oakley, Ca. USA Posts: 137	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..... __________________ 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
71GMC_3/4T Weapons Of Construction Join Date: Apr 2000 Location: Colorado, USA Posts: 1,095	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. __________________ 1971 GMC 2500, 402/TH400 4.10 Daily Driver Lafayette, CO

06-05-2002, 02:18 PM	#5
ron68 Registered User Join Date: Apr 2001 Location: galesburg, illinois US Posts: 40	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.

06-05-2002, 02:25 PM	#6
kpf Registered User Join Date: Jun 2002 Location: Camas, Wa Posts: 58	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
Spoof Registered User Join Date: Jul 2000 Location: Victoria,BC, Canada Posts: 1,300	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. __________________ 71 GMC 1500 LWB

06-05-2002, 03:49 PM	#8
tom hand CCRider Join Date: Feb 2000 Location: Olive Branch,MS,USA Posts: 2,232	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. __________________ 72 GMC Sierra SWB almost finished---- 84 Softail Olive Branch MS

06-05-2002, 10:30 PM	#9
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	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.

06-05-2002, 10:51 PM	#10
70shortnphat "See the U.S.A....." Join Date: Apr 2002 Location: Richmond TX. Posts: 269	definately change the shoes. __________________ 1970 short bed fleetside 1949 all origional chevy 3600 1974 camaro Richmond , Texas(just S.W.of Houston)

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