Jumps in Reverse - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**dagnabbitt** · 05-03-2021, 09:25 PM

I just took my 70 swb out of hibernation. Power steering fluid and oil were down and battery is on its last legs. What concerns me though is when I put it in reverse it jumps a few inches backwards even with the brake on. Does not do that when moving forward. Hoping my rear diff isn't shot, looking for a diagnosis and advice. Thx in advance!

body bolt · 05-03-2021, 09:29 PM

A stuck or dragging brake? Automatic or manual transmission? If auto did you check the fluid?

**dagnabbitt** · 05-03-2021, 10:24 PM

Quote:

Originally Posted by body bolt

A stuck or dragging brake? Automatic or manual transmission? If auto did you check the fluid?

It'd an automatic, don't think it is a brake issue. Will check the fluid level, thank you.

**LockDoc** · 05-03-2021, 11:36 PM

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Does it kill the engine if you put the brake on in reverse? My '39 Ford Coupe did that and it was a vacuum leak in the line to the modulator. 350/TH350. Check all of your vacuum hoses....

LockDoc

**dagnabbitt** · 05-04-2021, 12:04 PM

Quote:

Originally Posted by LockDoc

-
Does it kill the engine if you put the brake on in reverse? My '39 Ford Coupe did that and it was a vacuum leak in the line to the modulator. 350/TH350. Check all of your vacuum hoses....

LockDoc

No, it doesn't want to stall, it just flies backward

**LockDoc** · 05-04-2021, 01:00 PM

Quote:

Originally Posted by dagnabbitt

No, it doesn't want to stall, it just flies backward

I would still check all the vacuum lines and the modulator.

LockDoc

**Jiggers72-GMC** · 05-04-2021, 08:04 AM

If it hits hard in reverse, maybe the idle is too high??

**dagnabbitt** · 05-04-2021, 12:04 PM

Quote:

Originally Posted by Jiggers72-GMC

If it hits hard in reverse, maybe the idle is too high??

I was thinking that, but wouldn't it do the same moving forward?

**Jiggers72-GMC** · 05-04-2021, 12:32 PM

Maybe a bad motor mount ?? .....

72c20customcamper · 05-04-2021, 12:40 PM

My guess is brakes . Reverse is a very low gear as opposed to forward gears . So it can overpower the brakes if they aren't working properly.

**Accelo** · 05-04-2021, 03:42 PM

Sometimes the slack in the differential spider gears can cause slack in the differential.
Looks like excessive backlash in the gears but it's not. It can jump when the slack it taken up.
A lot of good suggestions on things to check on. This Board is a wonderful resource.
Rick

weq92f · 05-04-2021, 03:56 PM

.

I'm thinking brakes. Imagine if the hardware in the rear needs adjustment or is just worn out. Add a little slack in the gears the rear may not have much rear brake force applied when you place it in R. With the R being lower geared and nothing stopping the rear axles it could pull the truck back a bit.

Try putting your parking/hand/foot brake on as hard as you can before shifting to R.

-klb

**dagnabbitt** · 05-04-2021, 05:31 PM

Thanks for all the suggestions, I will be checking my vacuum hoses and modulator, but the brake question is a good one too, it has 4 old drums on it and it might be as simple as that. Thanks again everyone I appreciate it.

05-03-2021, 09:25 PM	#1
dagnabbitt Senior Member Join Date: Dec 2014 Location: Edmonton Alberta Posts: 1,475	Jumps in Reverse I just took my 70 swb out of hibernation. Power steering fluid and oil were down and battery is on its last legs. What concerns me though is when I put it in reverse it jumps a few inches backwards even with the brake on. Does not do that when moving forward. Hoping my rear diff isn't shot, looking for a diagnosis and advice. Thx in advance! __________________ DAVE Edmonton, Alberta 1959 Apache 1967 K20 1968 C20 1970 C10 1972 GMC 2500 1981 C10

05-03-2021, 09:29 PM	#2
body bolt Registered User Join Date: Oct 2020 Location: Albuquerque, New Mexico Posts: 474	Re: Jumps in Reverse A stuck or dragging brake? Automatic or manual transmission? If auto did you check the fluid? Last edited by body bolt; 05-03-2021 at 09:35 PM.

05-03-2021, 11:36 PM	#4
LockDoc The Older Generation Join Date: Aug 2006 Location: Montezuma, Iowa Posts: 25,896	Re: Jumps in Reverse - Does it kill the engine if you put the brake on in reverse? My '39 Ford Coupe did that and it was a vacuum leak in the line to the modulator. 350/TH350. Check all of your vacuum hoses.... LockDoc __________________ Leon Locksmith, Specializing In Antique Trucks, Automobiles, & Motorcycles (My Dually Pickup Project Thread) http://67-72chevytrucks.com/vboard/s...d.php?t=829820 -

05-04-2021, 08:04 AM	#7
Jiggers72-GMC Senior Member Join Date: Mar 2021 Location: up North Posts: 550	Re: Jumps in Reverse If it hits hard in reverse, maybe the idle is too high?? __________________ Gerry, North Bay Ontario 1971 Chevy K10 short stepper, 518 med green & white 1972 GMC C1500 long box, dark Brown, owned since 1977 1969 GMC C2500 manual

05-04-2021, 12:32 PM	#9
Jiggers72-GMC Senior Member Join Date: Mar 2021 Location: up North Posts: 550	Re: Jumps in Reverse Maybe a bad motor mount ?? ..... __________________ Gerry, North Bay Ontario 1971 Chevy K10 short stepper, 518 med green & white 1972 GMC C1500 long box, dark Brown, owned since 1977 1969 GMC C2500 manual

05-04-2021, 12:40 PM	#10
72c20customcamper Registered User Join Date: Jul 2018 Location: Catskill Mountains,NY Posts: 8,791	Re: Jumps in Reverse My guess is brakes . Reverse is a very low gear as opposed to forward gears . So it can overpower the brakes if they aren't working properly. __________________ Mark 72 c20 custom camper Husky edition, 66 SS396 Chevelle 1964 Hawk, 63 Avanti,62 lark 1969 AMX , 1968 c20 stepside ,85 K20 1977 Suburban sold 68 anniversary.

05-04-2021, 03:42 PM	#11
Accelo Senior Member Join Date: Mar 2015 Location: washington Posts: 2,321	Re: Jumps in Reverse Sometimes the slack in the differential spider gears can cause slack in the differential. Looks like excessive backlash in the gears but it's not. It can jump when the slack it taken up. A lot of good suggestions on things to check on. This Board is a wonderful resource. Rick

05-04-2021, 03:56 PM	#12
weq92f Registered User Join Date: Oct 2016 Location: Dallas Texas Posts: 3,267	Re: Jumps in Reverse . I'm thinking brakes. Imagine if the hardware in the rear needs adjustment or is just worn out. Add a little slack in the gears the rear may not have much rear brake force applied when you place it in R. With the R being lower geared and nothing stopping the rear axles it could pull the truck back a bit. Try putting your parking/hand/foot brake on as hard as you can before shifting to R. -klb __________________ 67 C10 fleet fuel injected '70 402, 700r4, 3.73 posi 07 335 sport turbo 6sp 94 Trans Am GT LT1 6sp posi -- sold after 22yrs 99 540 sport V8 6sp -- sold 73 240z L24 4sp -- given to friend 68 C10 step 350/350 3.73 open -- sold

05-04-2021, 05:31 PM	#13
dagnabbitt Senior Member Join Date: Dec 2014 Location: Edmonton Alberta Posts: 1,475	Re: Jumps in Reverse Thanks for all the suggestions, I will be checking my vacuum hoses and modulator, but the brake question is a good one too, it has 4 old drums on it and it might be as simple as that. Thanks again everyone I appreciate it. __________________ DAVE Edmonton, Alberta 1959 Apache 1967 K20 1968 C20 1970 C10 1972 GMC 2500 1981 C10



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