'71 M792 Brakes won't stop this thing!

[MinnesotaFirefighter]

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]

You rebuilt what?

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

 

[MinnesotaFirefighter]

You rebuilt what?

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

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]

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]

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.

 

[MinnesotaFirefighter]

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]

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]

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]

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

It is indeed a split system. One side feeds the center axle, while the other supplies both the front and rear axles.

 

[doghead]

Thanks Joe, yes I'm slipping...

 

[1958 M274]

Thanks Joe, yes I'm slipping...

You still know how to get to the GA Rally, right? See you in a few days?

 

[doghead]

Yes, I should be there on Wed.

 

[MinnesotaFirefighter]

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]

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

I'm sending you my phone number if you want to call.

 

[Superdave]

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.

 

[JR Scout]

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.