brakes locking up need help

[fultonfire]

i have a 1984 m923 that we use as our brush truck. the other night i was backing it into the fire station after my weekly drive to make sure it is running okay. anyways i was honking the horn while backing up and the truck started straining then all of a sudden the brakes locked up hard, the truck would not budge. i left it oustside overnight and went back the next day started it up and it moved fine so i took it for a drive. i got about 2 blocks then i could feel the brakes starting to apply so headed back to the station when i pulled in all back brakes were very hot ,to the point of them smoking. when i push the brake pedal i hear very little air when i release them and no sound when i work the parking break lever. i know nothing about air brakes and could use any help given. thanks for any help. the truck has abs sticker on windshield if that helps any

 

[TedG]

On deuces and 800 series and perhaps 900 series, you need to lubricate the air pack. go to the manual section of ss and down load the pertinent manuals. My M813A1 started doing this when I bought it and lubricated it worked for me, not to mention checking all the brakes, etc.

 

[fultonfire]

ok i looked at the TMs but never saw where the air pack is located is it under the pedal itself im sure i sound like a dummy but like i said i know nothing about air brakes if they were hydraulic i would know a little more thanks for the help.

 

[Irv]

Don't be embarrassed about being new to this! We have all been there. I had the same thing happen to me on a M813. The previous post is correct about the problem. There is a little hex head set screw on the back of the airpack, also known as an air assist brake. The pedal pushes on a standard brake master cylinder. That cylinder sends pressure down to a brake booster (airpack), where it is used to valve the full 100 psi of the air system into a 4 inch (or so) diameter piston/cylinder, which then forms up a tremendous pressure on the secondary hydraulics that then distribute pressure to all of the wheels. That airpack, or brake booster, needs lubing and many people don't know it is even there. Lots of the set screws appear to have never been opened. Use 10 weight air tool oil, available at any auto parts store. It only takes a few squirts out of a squirt can (best way to apply it, in fact). I think the dash 20 manual describes this procedure. If not, it would be in the lube order. That set screw may be rusted in place, so get a tight fitting allen head wrench on it and don't let it round the corners off. It will be hard to get out the first time.

In the future, if that happens, a field expedient trick is to slightly crack open one of the air valves in the front of the truck, right behind the bumper. There are glad hands there for when the truck is being towed. By using them, the towing truck can operate the brakes of it's towed load. This does not negate the need to lube the air pack, though.

Good luck. It's not hard and doesn't cost anything. You'll be on the road in a matter of minutes.

Irv

 

[DaveP]

seems to me that this truck uses a full air system?

 

[fultonfire]

thanks for all the help i will look at it when i get off work. i will try everything to fix this truck beacuse it is our only brush truck here and everything it very dry just hope the brush fires wont start till this rig is going. thanks for all the help and ill let yall know the outcome.

 

[fasttruck]

Like DaveP said 900 series trucks have straight airbrakes and all this about hydrovacs does not apply. If brakes on one axle do not release check quick release valve for that axle (mounted on frame above axle) for operation. this is where the air vents to hte atmosphere when brakes are released. when you left it sit overnight this air caught between the applicator valve and the quick release leaked off eventually and allowed brake to release. first time brake was used refilled lines and chamber and thus sticking brakes. When brake is stuck, chock truck and take a wrench underneath and loosen coupling at quick rlease valve and see if air comes out. Fix: replace quick release valve.

 

[Irv]

My mistake. I missed the part about it being 900 series. Yes, those are full air. My comments about the 800 series and the earlier 5 tons might be useful to someone with the earlier trucks. Irv

 

[fultonfire]

thank you going to station too look at it now

 

[fultonfire]

okay i looked at the truck and i found a quick release under the driver door step behind a acess plate when i bleed it off the truck will do fine until i hit the brakes a couple times then bam locked up again .where do i find new parts for this truck? does this release work all the wheels? i looked at a tm that had this one in it just not sure which tires it works.could not find the ones over the axles. if it has 2 at the back of the truck would both go out at once for safety?

 

[fultonfire]

still have brake problems i replaced the quick release and checked the two valaves above axles and they work.my lights of low air does not work nor does the park break light. if anoyone can think of anything else to check please let me know because i feel like pulling out a 10 pound sledge hammer on it

 

[akonitony]

May I recommend a 12# sledge minimum...

What about the kinuter valve? Have you tried adjusting that?

 

[Rattlehead]

Do you have the manuals for this? TM 9-2320-272-10 has general descriptions of all the components, and TM 9-2320-272-24-4 has all the air line schematics. They are very confusing.

One thing comes to mind is that when you apply the parking brake, it not only operates the mechanical drum brake on the back of the transfer case, it also has an air valve that dumps the spring brake pressure on the rear axles, applying the spring brakes. Possibly this valve is leaking off air? Next time it happens, pull the dust cover out of your spring brake and see if the piston is way inside or if it is close to the hole. If it is deep inside (2"?) then the spring brakes are applied and you are leaking off that air somehow.

Do you think the front brakes are not applying at all?

I also forgot, there is a spring brake release valve on the dash. It is there so that you can set the mechanical parking brake, but then push in this valve to release the spring brakes.

You should make sure that you have T-bolts for backing off your spring brakes for emergencies or breakdowns so it can be towed. There is a storage spot on each of the spring brakes for them, make sure they're in place or get replacement ones.

I assume that you are not losing all of the truck's air pressure when this happens? Which of course would apply the spring brakes.

 

[Rattlehead]

Any luck with this?

Do you know if your truck has the ABS addition?

I thought some more about this, and there is a limiting valve that allows the rear brakes to be applied first, just like on a car with hydraulic brakes. If you are getting pressure to the brake system, it might only affect the rears. Maybe your treadle valve (the brake pedal) is bleeding air pressure into the apply circuit? You could loosen a signal line on one of the rear relay valves and see if it is getting pressure when the brakes are not being applied.

 

[Maverick1701]

Any luck with this?

Do you know if your truck has the ABS addition?

I thought some more about this, and there is a limiting valve that allows the rear brakes to be applied first, just like on a car with hydraulic brakes. If you are getting pressure to the brake system, it might only affect the rears. Maybe your treadle valve (the brake pedal) is bleeding air pressure into the apply circuit? You could loosen a signal line on one of the rear relay valves and see if it is getting pressure when the brakes are not being applied.

IIRC I recall reading somewhere that his truck has the ABS sticker

 

[Construction]

I know this is an old post to respond to, but Irv - where is the hex screw you are talking about? I have an air pack un-installed and cannot find it?