M939, no spring (parking) brakes

[m16ty]

I just picked up my M936 from GL today. Everything works fine except the spring brakes. I can't get them to operate at all. The funny thing is they are in the released position. In other words, with zero air pressure in the truck the spring brakes are still released. Working the parking brake handle or the button on the dash does nothing. The parking brake handle will operate the manual parking brake on the t-case.

The truck will drive like this but in this condition if you loose air pressure, you will have zero brakes.

Any ideas?

 

[212sparky]

are they caged?

 

[quickfarms]

can you post pictures of the rear air cans?

 

[m16ty]

are they caged?

No.

 

[212sparky]

i have seen the cans come apart before from the spring binding up. be careful if you tru to split the cans to look in them.

 

[EMD567]

I'll bet the brakes are caged. Look on the cans- there should be a bolt with an arrow- caged or uncaged. Normal operation is uncaged.

 

[quickfarms]

Pull the dust covers off and see if the brakes are properly adjusted.

The dash valve could be bad.

I would have to go throught the air diagram to see if the dash valve controls the spring brakes or is there a relay valve in the system that could be stuck.

Drain the air tanks and start the truck and try to move it before it builds air pressure.

 

[m16ty]

I'll bet the brakes are caged. Look on the cans- there should be a bolt with an arrow- caged or uncaged. Normal operation is uncaged.

I figured they were like all other brake chambers I've been around. All I've seen you have to insert a bolt into the hole in the back and tighten it to cage the brakes. Are you saying these are different? There is no bolt inserted in the back.

I'm trying to find the -34 manual to try and get a brake diagram. Don't see them on the site and LOGSA appears to be down.

 

[m16ty]

I did. It rolls freely.

 

[gimpyrobb]

 

[jaxsof]

Maybe some "special" individual put a front-rear in place of a rear-rear, and there are no maxi chambers onthis vehicle. If they are there, they dont look like the chambers on axle 2. The can is roughly twice as big on the third axle, well, its supposed to be, anyway.

(and as far as I can see, the -34 manuals are not on logsa, but google books has 34-1, from 1982)

 

[m16ty]

The maxi chambers are indeed there. All I can figure is somebody adjusted them off so they wouldn't work. By looking in the hole where the caging bolt goes the chamber is applied but the brakes aren't.

I've fooled with air brake trucks all my life but I've never worked on wedge type before. I also find it odd that there are two chambers on each wheel. I'm sure I can figure it out. I just need to find out how the system works.

 

[quickfarms]

Are the rear brakes actually applying at all?

I remember having issues with wedge brakes when the shoes were worn, the drums worn and they were not properly adjusted.

 

[nf6x]

(and as far as I can see, the -34 manuals are not on logsa, but google books has 34-1, from 1982)

LOGSA has the -24 manuals, which cover everything except depot repair. The way those numbers like -10, -20, etc. work is that the first digit indicates the lowest repair echelon covered, and the second digit indicates the highest (or 0 if the manual covers only one echelon). So, a -24 covers everything that would be in a -20, -30 or -40 manual.

Those older -34 manuals would have coexisted with -20 manuals, but newer -24 manuals would supersede both the -20 and -34 manuals.

Did that make any sense at all?

 

[doghead]

We have the TMs here on SS. http://www.steelsoldiers.com/5-ton-up-tms/

The "pistons" that spread are known to stick on wedge brakes. Yours could all be stuck.

The parts TM shows both types of chambers used.

 

[m16ty]

Are the rear brakes actually applying at all?

I remember having issues with wedge brakes when the shoes were worn, the drums worn and they were not properly adjusted.

Shoes appear to be brand new. The service brakes work fine.

We have the TMs here on SS. 5-ton and up TMs - Steel Soldiers::Military Vehicles Supersite

The "pistons" that spread are known to stick on wedge brakes. Yours could all be stuck.

The parts TM shows both types of chambers used.

I guess the pistons could be stuck. It seems if they were to stick from sitting they would be stuck in the "on" position though.

Yes, both chambers are used. On the front of the axle there is a maxi (spring) chamber. On the back there is just a single chamber. From what I can tell so far, the single chamber is what works your service brakes and will apply when you push the brake pedal. It seems the maxi chamber is only used for parking/emergency brakes.

I've looked in the -24 manual but couldn't really find the info I wanted. That's why I was looking for a -34 manual. I'm very familiar with air brake principles and have built air brake systems before. I'm just not familiar with this system. A little bit of tracing lines and I should be able to figure it out.

IMO, any of the TMs do a great job of how to perform a task, step by step. They don't really dig into the how or why though. For somebody like myself that is familiar with how air brakes work, the how and why would really be helpful. I'm sure I can figure it out on my own, it's just going to take a lot longer.

 

[groundog]

there is a site called liberated manuals---just need tm number

 

[nf6x]

IMO, any of the TMs do a great job of how to perform a task, step by step. They don't really dig into the how or why though. For somebody like myself that is familiar with how air brakes work, the how and why would really be helpful. I'm sure I can figure it out on my own, it's just going to take a lot longer.

Agreed. I've found that the tech manuals for mil radios were much better for a collector like me prior to the advent of field-swappable modules that would only be repaired at depot level. For the old tube-based, non-modular radios, the tech has to really understand how the radio works in order to do component-level repair. So, their manuals include complete schematics and very detailed theory of operation sections.

Newer radios are much faster and easier to fix in the field, because the tech just needs to troubleshoot down to a faulty module, then swap the module. The manuals no longer need to include component-level schematics, and the theory of operation sections don't need to go into as much detail. The TMs are still thicker since the radios are so much more complicated, but they don't go as deep into the circuitry. This is a lot better for military use with a full logistics and support chain, but not so helpful for a collector who's everything from the -10 operator through the -50 depot.

Back on topic now... I'm watching this with interest since I'll be getting my first experience with an M9xx truck soon, and I haven't worked on air brakes before. These symptoms don't sound like the failure mode that I'd expect, either.

 

[doghead]

I'm sorry, I missed the fact that the -34 TM is not here(thought it was).

Did you try Jatonka's site?

 

[quickfarms]

From what I remember about wedge brakes, it has been over 20 years, the front and rear shoes are adjusted independently. A Maxi can is a dual brake chamber, it combines both the service brake and the spring brake. It is possible that the spring brakes were adjusted off and the service brake are working but only half capacity