M939, no spring (parking) brakes

[jasonjc]

If there is a -24 there will not be a -34. As for how the brakes work, i will be watching this.

 

[73m819]

you can have single OR double pot wedge brakes, the single work just like a S-cam (the single wedge spreads both shoes), spring brakes most times do NOT have any adjustment eithe s-cam or wedge, either applied or not, it releies on the spring and normal service brakes to hold, the two pot wedge brakes work both shoes for the service brakes (one for each ), so the way the 900 5ts are set up, only ONE shoe is used on each end of the axles that has a spring brake

 

[quickfarms]

If the service brakes are not correctly adjusted it will affect the spring brakes

 

[73m819]

If the service brakes are not correctly adjusted it will affect the spring brakes

True, though the brakes have to be SO out of adjustment that thay will NOT stop the wheel. the OP said that the service brakes worked fine, even if one wheel was that far out, I do not see how ALL the wheels can be that far out of adjustment, if this was the case, there would be major stopping proplem

 

[quickfarms]

An empty truck would stop just fine using only the front axle brakes.

It is possible that the spring brake portion of the rear brakes were adjusted so that the truck could be moved without canning the brakes.

If this is possible that half of the rear brakes would still work.

When I dealt with dual can wedge brakes each shoe had it own adjuster. There were two adjusters per wheel.

 

[73m819]

very true but it would seem it would be easier to cage. not sure of the style of adjusters, some wedge have a adjuster like the older 5ts, others have a crappy adjuster that adjust at the shoe toe

 

[quickfarms]

Maybe they did not have the caging bolts or could not get them off of the cans.

 

[m16ty]

The M939 trucks have two chambers per wheel,not just a double chamber. One chamber is a single and is for the service brakes. The other chamber is a double chamber with spring brakes and is for emergency/parking. I think that is where some people are getting confused about the problem I'm having. On the M939 trucks there are actually 4 adjusters per wheel, one for each chamber.

The service chamber works fine and is a completely separate system from the parking brakes. From the research I've done, unlike any vehicle I've ever worked on before, the service and parking systems are completely separate.

I think I've finally figured out how the system works. Now it's just a matter of tracing lines and testing valves to find out where the problem is.

 

[m16ty]

not sure of the style of adjusters, some wedge have a adjuster like the older 5ts, others have a crappy adjuster that adjust at the shoe toe

They have a "star" nut type adjuster on the plungers that contact the shoe. Kind of like an old pickup hyd brake adjuster. TM says you need a tool but it looks like a flat bladed screwdriver would work. Doesn't look real easy to do through a small hole in the backing plate.

I wish they had went with regular "S" cam brakes and slack adjusters. That would be much easier to service IMO.

 

[73m819]

They have a "star" nut type adjuster on the plungers that contact the shoe. Kind of like an old pickup hyd brake adjuster. TM says you need a tool but it looks like a flat bladed screwdriver would work. Doesn't look real easy to do through a small hole in the backing plate.

I wish they had went with regular "S" cam brakes and slack adjusters. That would be much easier to service IMO.

You can use a standard brake adjusting tool that was used on the older cars with the star adjustment, the bad part is at times things freeze up like the ones on cars but about 10 tines worse, real easy to strip the nubs off the adjuster, then you are SCREWED, or adjust out to touch the drum, then back off to set drag, BUT the fun begins when it WILL NOT back off, SCREWED again because now you have to FIGHT to get the drum off to fix the F thing

 

[Bolkbich]

subscribed

 

[73m819]

The M939 trucks have two chambers per wheel,not just a double chamber. One chamber is a single and is for the service brakes. The other chamber is a double chamber with spring brakes and is for emergency/parking. I think that is where some people are getting confused about the problem I'm having. On the M939 trucks there are actually 4 adjusters per wheel, one for each chamber.

The service chamber works fine and is a completely separate system from the parking brakes. From the research I've done, unlike any vehicle I've ever worked on before, the service and parking systems are completely separate.

I think I've finally figured out how the system works. Now it's just a matter of tracing lines and testing valves to find out where the problem is.

You have a duel pot system, ONE pot for EACH shoe, in this case using a single pot for one shoe and a double pot for the other, the double pot contains the standard pot (same as the other shoe) to apply the service brakes, the second pot (spring brake) contains the SPRING that TAKES OVER for the lack of air to apply that shoe only. air pressure keeps the spring brake in affect gaged, the lack of air allows the spring to put pressure on the actuating rod and in turn set the brake just as if you stepped on the brakes

There will be TWO compleat air systems, lines, valves, relays, quick release, ect., and may be some safety interlocks

 

[m16ty]

I finally figured out what the problem was. First off all the brakes were bad out of adjustment. The main problem though was a maxi chamber screw up on the drivers side front tandem. This wheel just happened to be the one I jacked up and was having the problem with the spring brake not applying. All the other spring brakes were working.

What I found out on the problem wheel is either a rookie mechanic had been working on it or somebody that didn't care. Upon further investigation here's what I figure happened- Somebody replaced the maxi chamber on that wheel. When they replaced it they had the emergency and service lines crossed. After replacement they then tried out the brakes and that axle wouldn't release. Instead of trying to find out what the problem was, they just adjusted the brake shoe all the way off and called it a day.

When I got to inspecting the problem wheel I found the emergency and service lines crossed and the brake adjuster adjusted all the way off. After a swap of the lines and a brake adjustment, everything was working as it should.

I will add that brake adjustment on the M939 trucks is a pain in the a**.

 

[juanprado]

Did you find the-34 manual and could you post it?

Thanks,

Juan

 

[m16ty]

I'm not sure there is a -34 manual for the M939. I haven't found one anywhere or any reference to one. The -24 manual pretty well covers everything and is more in depth than the -24 manuals on other trucks.

I tend to think they put all the info in the -24 for this series.

 

[nf6x]

Yes, that is how TM numbering works. If there is -24 manual, then there won't be a -34. Or a -20, -30, -40, or -23, for that matter. The -24 covers everything from the second repair echelon (unit support) through fourth echelon (general support).

I'm glad that you found your problem, and the related discussion was educational!

 

[rickyrecon]

Help with Spring Brake - M939

I have a 1985 M939 and the guy I bought the truck said the spring brake wasn't working... bought the truck anyway only to find that someone had damaged it with a crow bar as if they were trying to get into the protective cover around the drive shaft and removed it. Needless to say - it needs to be replaced as I have no emergency brake and have a lot of rattling around inside the banged up protective cover.

I have no idea where to buy the part and if I'm capable to repair it myself. Any advice is greatly appreciated or - I'm willing to pay someone one to help me fix it or fix it for me.

 

[98G]

I have a 1985 M939 and the guy I bought the truck said the spring brake wasn't working... bought the truck anyway only to find that someone had damaged it with a crow bar as if they were trying to get into the protective cover around the drive shaft and removed it. Needless to say - it needs to be replaced as I have no emergency brake and have a lot of rattling around inside the banged up protective cover.

I have no idea where to buy the part and if I'm capable to repair it myself. Any advice is greatly appreciated or - I'm willing to pay someone one to help me fix it or fix it for me.

PM sent with contact info. I have some pics for you....

 

[rickyrecon]

My mistake... I believe it's the Emergency Brake that is tore up and not the Spring Brake as the protective cover that is damaged is over the drive shaft. I can feel two brake shoes in there where the protective cover has been pulled back with a crowbar. One shoe and is loose and just floating around. Any advice is appreciated.

 

[98G]

My mistake... I believe it's the Emergency Brake that is tore up and not the Spring Brake as the protective cover that is damaged is over the drive shaft. I can feel two brake shoes in there where the protective cover has been pulled back with a crowbar. One shoe and is loose and just floating around. Any advice is appreciated.

When your park brake lever is in the park position the air should dump out of the emergency side of the brake system and your springbrakes should apply. The springbrakes are plenty to hold the truck by themselves (not saying you shouldn't fix your park brake as well, but you have not yet found the entirety of the problem ).

Are your brakes caged? (Do you know what caged brakes look like?)

Are you using the override on the dash?