Factory 12 bolt posi question - The 1947 - Present Chevrolet & GMC Truck Message Board Network

murguia · 12-17-2023, 02:30 AM

Just converted my 3 speed to a turbo 350. As I was wrapping everything up and and the truck was on the lift, I put the truck in gear and pressed the brake firmly but the right rear wheel was spinning.

Question, how would only one whee spin on a posi?

I’m hoping it’s just a brake issue (cylinder, adjustment, leak…) in that back rear wheel. I will say the brakes don’t feel right, like they did before I did all the work. The pedal is almost bottoming out when I’m completely stopped.

TIA

**ohboy321** · 12-17-2023, 06:02 AM

I can't comment on your possible brake issue, but one reason for a posi to behave like a non posi is if there is an issue with the clutches. I did not know I had a posi until I pulled the cover and chunks of clutch plates fell out. I had used the spin the wheel test prior to finding/fixing that and I thought I had a one legged axle.

ghackett1 · 12-17-2023, 07:51 AM

Quote:

Originally Posted by murguia

Just converted my 3 speed to a turbo 350. As I was wrapping everything up and and the truck was on the lift, I put the truck in gear and pressed the brake firmly but the right rear wheel was spinning.

Question, how would only one whee spin on a posi?

I’m hoping it’s just a brake issue (cylinder, adjustment, leak…) in that back rear wheel. I will say the brakes don’t feel right, like they did before I did all the work. The pedal is almost bottoming out when I’m completely stopped.

TIA

FWIW: Did you decode the rear end? Not all 12 bolts are posi.

**bowtie72** · 12-17-2023, 08:11 AM

Sounds like you have a brake problem first. Pull the right wheel and drum and inspect the shoes and hardware. I would pull the left also. It may be worn out if it’s the only one working.

**1970cstblazer** · 12-17-2023, 08:16 AM

The factory "posi" is a limited slip, not locker(NO-SPIN 20-30 series) unless stated as such on the SPID. There will be slippage under certain conditions.

murguia · 12-17-2023, 09:22 AM

Thanks everyone. I changed t he gear oil yesterday and it looked great inside. Drove it home on the freeway and it cruised great.

Up in the air I spun one and both spun the correct direction.

I used maps 80-90 gear oil and added the additive they told me.

I’ll start with checking the brakes

Thanks

ghackett1 · 12-17-2023, 03:20 PM

Quote:

Originally Posted by murguia

Thanks everyone. I changed t he gear oil yesterday and it looked great inside. Drove it home on the freeway and it cruised great.

Up in the air I spun one and both spun the correct direction.

I used maps 80-90 gear oil and added the additive they told me.

I’ll start with checking the brakes

Thanks

Great news! Brakes much easier and cheaper LOL

12-17-2023, 02:30 AM	#1
murguia Registered User Join Date: Apr 2007 Location: la Posts: 369	Factory 12 bolt posi question Just converted my 3 speed to a turbo 350. As I was wrapping everything up and and the truck was on the lift, I put the truck in gear and pressed the brake firmly but the right rear wheel was spinning. Question, how would only one whee spin on a posi? I’m hoping it’s just a brake issue (cylinder, adjustment, leak…) in that back rear wheel. I will say the brakes don’t feel right, like they did before I did all the work. The pedal is almost bottoming out when I’m completely stopped. TIA

12-17-2023, 06:02 AM	#2
ohboy321 Senior Member Join Date: Aug 2015 Location: Dunbarton, NH Posts: 283	Re: Factory 12 bolt posi question I can't comment on your possible brake issue, but one reason for a posi to behave like a non posi is if there is an issue with the clutches. I did not know I had a posi until I pulled the cover and chunks of clutch plates fell out. I had used the spin the wheel test prior to finding/fixing that and I thought I had a one legged axle. Attached Images __________________ Steve/NH 1967 K10 327 Short Bed Stepside 2008 F350 Power Stroke Lariat 2007 BMW X3 2023 BMW X3 M-Sport 2020 BMW 540xi M-Sport 1995 Harley FLSTC

12-17-2023, 08:11 AM	#4
bowtie72 Registered User Join Date: Mar 2007 Location: Colorado Posts: 362	Re: Factory 12 bolt posi question Sounds like you have a brake problem first. Pull the right wheel and drum and inspect the shoes and hardware. I would pull the left also. It may be worn out if it’s the only one working. __________________ 1968 SWB "Little White Truck" 1972 LWB LS 4l80 "Plain Jane!!"

12-17-2023, 08:16 AM	#5
1970cstblazer All stock and staying that way Join Date: Jul 2003 Location: Elkland, PA Posts: 1,676	Re: Factory 12 bolt posi question The factory "posi" is a limited slip, not locker(NO-SPIN 20-30 series) unless stated as such on the SPID. There will be slippage under certain conditions. __________________ 1970 K/5 Blazer CST 4WD, Medium Bronze, 93k ACT. miles, 350, 4 speed, rear positraction, 16.5" x 8.25" HD wheel option, tilt, tach, vacuum, AM/FM, manual throttle...Dad ordered and purchased new 4/70. Currently frame off restoration finally getting close to completion.. 1972 Cheyenne Super 20 2WD, DK Blue/White, 90K ACT. miles, 402, TH400, 4.10 open, tilt, tach, vacuum, A/C, AM/FM, manual throttle.. A mostly original paint never rusted Texas survivor... 2017 Sierra 1500 SLT 4WD, Black, 45k miles, 5.3, 6L80E, 3.42 LS, 20" polished wheels, everything but moonroof and 6.2... 2019 Canyon SLT 4WD, White, 62k miles, 3.6, 8L45, 3.23 LS, 18" wheels

12-17-2023, 09:22 AM	#6
murguia Registered User Join Date: Apr 2007 Location: la Posts: 369	Re: Factory 12 bolt posi question Thanks everyone. I changed t he gear oil yesterday and it looked great inside. Drove it home on the freeway and it cruised great. Up in the air I spun one and both spun the correct direction. I used maps 80-90 gear oil and added the additive they told me. I’ll start with checking the brakes Thanks Attached Images



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