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Author Topic: some questions on 12 bolt rear  (Read 8279 times)

Offline dvdswan

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some questions on 12 bolt rear
« on: March 17, 2014, 11:35:09 pm »
so the last couple of days the rear has started to lock up the tires when braking, so I'm thinking the axle seals or wheel cylinders are leaking.  anyway, I was going to take a look at it this weekend and was going to service the diff with the factory LSD. 

what would be a good recommended oil for the diff and additive for the 12 bolt?  is there anything I should look for in the rear diff (as far as worn parts)?  I'm sure the parts are readily available but are there any that are not?

thanks for the help.
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Offline bake74

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Re: some questions on 12 bolt rear
« Reply #1 on: March 18, 2014, 05:51:55 pm »
     80/90 weight is all you need for the dif.  Synthetic oil will not break down in viscosity as fast, but when it does break down it likes to turn to sludgy material (this take a ling time though ).
     As far as additives, I have never used any and never had problems, but I have always changed mine in the past frequently because of the type of 4 wheeling I was doing.
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Offline dvdswan

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Re: some questions on 12 bolt rear
« Reply #2 on: March 18, 2014, 09:42:13 pm »
thanks bake.
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Offline Irish_Alley

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Re: some questions on 12 bolt rear
« Reply #3 on: March 18, 2014, 10:53:33 pm »
what makes you think they are leaking? do you see fluid dripping out of the drum? are you low on brake fluid?
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Offline dvdswan

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Re: some questions on 12 bolt rear
« Reply #4 on: March 19, 2014, 11:38:43 pm »
rear brakes have locked up a couple of times on light braking.
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Offline Irish_Alley

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Re: some questions on 12 bolt rear
« Reply #5 on: March 20, 2014, 04:01:07 am »
do you see any fluid around the drums? i do understand that wet shoes will stick but its been a long time since i've had a 12 bolt apart. but bad pads/drums will do this also. my 94 rear cylinder leaks enough to see it on the tire. it doesnt lock up. and you do have a rubber line on the rear wheels. it could collapse or get plugged with rust, to test that jack the rear up (in neutral) and have someone apply the brakes and try to spin the tire, then have them release the brakes. you should be able to spin it as soon as they release the pedal
« Last Edit: March 20, 2014, 04:09:28 am by Irish_Alley »
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Offline dvdswan

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Re: some questions on 12 bolt rear
« Reply #6 on: March 23, 2014, 08:08:00 pm »
well, I took the truck to chevy for a LOF and checkup on the 20th, turned out the master cylinder was bad/leaking (physically dripping) so I'm guessing the fronts weren't doing the work they should have.  had them replace it and the truck stops beautifully now.  got it home yesterday and figured I check the brakes and service the axles.

jacked up and on jack stands...


I guess the truck sat for a while before I got it...


um yeah, wonder how long the axle seal has been leaking... pass. side


dr. side... a little dirty


and that doesn't look like a LSD... WTH... well after cleaning the diff cover I found some paint pen on it, seems to be replaced from a junkyard... 


and after some cleanup... my heart was broken... 3.07s WTH I was hoping for at least 3.73s...  incidentally only about 6 ozs. of fluid cam out when I drained it and had a nice burnt smell, clean it up really good it had some sludge but not thick, I guess we will see if it starts to make some noise now...


buttoned up...


onto the front...


well at least they match, but I'm still wondering what happened to the G80 in the rear...


I don't think the diff cover had ever been off.  fluid looked ok thankfully, but it still needed to be serviced...


and all buttoned up...



« Last Edit: March 23, 2014, 08:17:46 pm by dvdswan »
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Offline pholliday1

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Re: some questions on 12 bolt rear
« Reply #7 on: March 23, 2014, 09:08:12 pm »
Alright. Not to be a party crasher but did you check the vent tube on the rear axle? A lot of times when an axle leaks you need to find out why. Everyone usually assumes it was the "seal". When checking the vent tube you can check the rubber hose with air, but when checking the axle housing you need to physically make sure the hole is open and not full of debris. Don't sweat the fact that your axle didn't have the g80 because honestly given the condition of the diff. It probably would've been junk anyways. And the bright side with an open diff you can easily add a locker! Good luck!
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Offline dvdswan

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Re: some questions on 12 bolt rear
« Reply #8 on: March 23, 2014, 10:36:18 pm »
I did check the breather tubes on both the front and rear, only because they were disconnected because of the lift.  apparently the PO didn't lengthen them so the ripped off the axles.  go figure.
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Offline bake74

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Re: some questions on 12 bolt rear
« Reply #9 on: March 24, 2014, 07:46:41 am »
     Just wondering, did you buy the truck being told it had the g80 in the rear ?  Also, I hate when my truck sit's for awhile and I go climb under it and there are spider webs everywhere.  I wonder why they like vehicles so much.
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Offline pholliday1

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Re: some questions on 12 bolt rear
« Reply #10 on: March 24, 2014, 09:46:50 pm »
Not to turn this into a spiderweb thread, but webs are to catch lunch and dinner I wonder if we have a bunch of critters climbing around our trucks LOL
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VETERAN AND LIFETIME NRA MEMBER. WHAT DO YOU STAND FOR?
1991 V3500 CREWCAB SRW 454TBI GMC
1991 v3500 crewcab SRW 454tbi 480le 6" lift
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1989 v3500 crewcab 454TBI 5" lift

Offline dvdswan

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Re: some questions on 12 bolt rear
« Reply #11 on: March 24, 2014, 11:11:55 pm »
Quote from: bake74 on March 24, 2014, 07:46:41 am
     Just wondering, did you buy the truck being told it had the g80 in the rear ?  Also, I hate when my truck sit's for awhile and I go climb under it and there are spider webs everywhere.  I wonder why they like vehicles so much.

not really, it was just an added plus.  the guy said it had a LSD and 4.11s, I looked at the build sheet and saw the G80 but nothing on the gears so I figured they were a 3.5 ratio or close too.  I bought the truck because of how original everything seemed to be.
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