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Tool Talk Discussion Forum

OT 65 GMC 4000 brakes

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Chris in MO

03-02-2008 02:46:10




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I've got a GMC 4000 truck. Recently when I step on the brakes, it feels a little mushy, but still stops. If I pump a few times, I've got some grab. When I step on the brakes, I also hear what sounds like it might be a vacuum leak. As a side item, within the last month the rear end has dropped out of low gear when I let up on the gas pedal. I don't use this truck much so it gets forgotten, but a discussion on the tractor talk board about brake failures reminded me that I need to do something about this before I get busy.

Any suggestions where to start looking?

Christopher

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dieselpaul

03-02-2008 08:01:54




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 Re: OT 65 GMC 4000 brakes in reply to Chris in MO, 03-02-2008 02:46:10  

hi chris-sounds like a couple of problems. first, ''sounds like vacumn leak when I step on brakes'' is got to be related to booster unit mounted under cab behind drivers step. you may hear it @ vent line behind seat when booster leaks internally. I believe your truck also uses a vacumn 2-speed shift motor, thus brake booster leakage could make it act up. you probly have air in system, also basic brake adjustment is very important on these old gm trucks-btw, there are 2 adjuster slots per each rear wheel, the top one has been known to make grown men cry--ha this truck has no back-up system on brakes[frt-rear seperated] so they require a good look-over hope this helps. paul

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fixerupper

03-02-2008 07:10:47




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 Re: OT 65 GMC 4000 brakes in reply to Chris in MO, 03-02-2008 02:46:10  
You might have two problems going on. The brake fluid is low and it has a loss of vacuum from a leak somewhere.

A weak brake booster shouldn't make the brakes mushy- it'll just make you have to push the pedal HARD to get brakes.

If the two speed axle is vacuum operated, then low vacuum might be the culprit there. Low vacuum might be caused by a leak in the brake booster? How does the engine idle? Does the engine idle rough when you use the brakes?

It's been quite a few years since I've worked on a truck with hydraulic brakes, but it seems like the brake booser tends to be a little problematic.

BigJT where are you? Jim

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Festere'

03-02-2008 06:54:37




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 Re: OT 65 GMC 4000 brakes in reply to Chris in MO, 03-02-2008 02:46:10  
When was the last time you adjusted these brakes?



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Bob

03-02-2008 04:04:02




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 Re: OT 65 GMC 4000 brakes in reply to Chris in MO, 03-02-2008 02:46:10  
Does it lose fluid?

Does the pedal ever go clear to the floor with the first "pump"?

Does the pedal ever get "hard"... you can't depress it any more, but the brakes aren't "grabbing"?

Does it have a "Hydrovac" vacuum brake booster mounted remotely from the master cylinder (to the frame rail)?



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