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'71 M792 Brakes won't stop this thing!

M

MinnesotaFirefighter

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Hey all. Just rebuilt the brakes in our Fire fighter M792 and bled the brakes as you would any brake system. The problem is the brakes will not even slow this machine down. I am pretty sure the system does not have any air left in it. DOT5 brake fluid used. Any help?
Thanks!
Al
 
doghead

doghead

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You rebuilt what?

Master cylinder, air-pack, wheel cylinders, shoes?
 
M

MinnesotaFirefighter

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doghead said:
You rebuilt what?

Master cylinder, air-pack, wheel cylinders, shoes?
Click to expand...
New brake shoes and rebuilt the wheel cylinders, Did not touch the air-pack.

I have good brake pedal but no stopping power. Was wondering if the shoes are not grabbing hard enough.
I don't know much about the air-pack or how it works. Since this is an Emergency response unit I want the brakes working properly ( Duhh) I know....
The brakes worked well until someone drove it with the hand brake on. That is when we had initial brake problems.
The Master Cylinder appears to be fine since the pedal is stiff and does not leak down.
Thanks!
Al
 
doghead

doghead

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Leaving the parking brake on should not affect the service brakes.

The air-pack is what "boosts" the brakes(like providing power brakes).

That's probably your problem.
 
doghead

doghead

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Oh crud, I just realized which forum/vehicle we're in.

Forget my previous info. (I was refering to 2.5 ton trucks)


I do own a goat, so I am familiar with your vehicle.

Goats do not have boosted brakes.
 
M

MinnesotaFirefighter

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OK, any ideas? Brake adjustment maybe? I get a litlle braking on the left side but "the oh crap factor comes in if you have to stop within a city block" I am looking for the tech manual so I can service this vehicle without being completely in the dark :)
 
doghead

doghead

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I just looked and our site only has the operators TM available.

I have all the TMs on my computer, but they are too big to send to you.

The brake system is "split", iirc each half of the master cylinder feeds one front wheel and two opposite side rear wheels.
 
1958 M274

1958 M274

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Doghead! You're slipping...

MinnesotaFirefighter,
The parking brake is a mechanical driveline brake and completely unrelated to the hydraulic service brakes. Driving with it on would have no affect on them, so you had another problem... What kind of "brake problems" did you experience?
 
1958 M274

1958 M274

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doghead said:
The brake system is "split", iirc each half of the master cylinder feeds one front wheel and two opposite side rear wheels.
Click to expand...
It is indeed a split system. One side feeds the center axle, while the other supplies both the front and rear axles.
 
doghead

doghead

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Thanks Joe, yes I'm slipping...
 
doghead

doghead

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Yes, I should be there on Wed.
 
M

MinnesotaFirefighter

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We lost pretty much all braking power. We thought the shoes were shot and they were down to nothing. Someone had driven the vehicle in 6wheel drive on the hard road and with the manual brake on. There was smoke rolling from under the vehicle. After rebuilding with new shoes on all 6 axles and new wheel cylinders and using a brake bleeder, we still have lousy braking. So bad NO ONE WANTS TO DRIVE IT. This is my Favorite vehicle for Grass and wildland fire. It's just too dangerous to leave the Fire house with it. I will start from ground ZERO with it if I have to. I want it back in service. It is too nice and too valuable to let sit.
Thanks!
Al
 
1958 M274

1958 M274

Member
333
4
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Location
North GA
MinnesotaFirefighter said:
We lost pretty much all braking power. We thought the shoes were shot and they were down to nothing. Someone had driven the vehicle in 6wheel drive on the hard road and with the manual brake on. There was smoke rolling from under the vehicle. After rebuilding with new shoes on all 6 axles and new wheel cylinders and using a brake bleeder, we still have lousy braking. So bad NO ONE WANTS TO DRIVE IT. This is my Favorite vehicle for Grass and wildland fire. It's just too dangerous to leave the Fire house with it. I will start from ground ZERO with it if I have to. I want it back in service. It is too nice and too valuable to let sit.
Thanks!
Al
Click to expand...
I'm sending you my phone number if you want to call.
 
Superdave

Superdave

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How were the brake drums, As if they were smoking as you say they could be glazed up and the new shoes would not be able to grip the drums as they would be very hard and like chrome. You would have to somehow machine the drums for one to get full shoe contact and to possibly remove the Glazed surface on the ones that got overheated. If you jack up one wheel and get someone to apply the brake can you move the tire at all ?? Just my two sense.
 
J

JR Scout

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Montgomery AL
Late to the party here, I know, but reading about brake problems on an Emergency response unit reminded me of the old joke about the VFD truck screaming into the middle of a big factory fire, frantically beating the flames out, then spending the reward money from the grateful factory owner on new brakes.
 
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