trailer air brake operation

[fleetmech]

I took a quick look under my truck the other day, and it looks like the modulated air supply to the trailer gladhand comes off the airpac, not the master or valve on the pedal system. Is it safe to assume then that a hydraulic circuit failure on the truck could also potentially stop the booster from sending air to the trailer brakes as well?

 

[cattlerepairman]

Here is the air line diagram:



So, what you are asking is "is the piston (4 and control valve piston 5) inside the compensator mounted to the airpack actuated by hydraulic pressure, air pressure or mechanically"?



Based on the assembly instructions, the control piston (5) protrudes into the bore of the slave cylinder. That would suggest that it is being activated by hydraulic pressure.



From rebuilding my air pack I do not remember the hydrualic piston inside the slave cylinder being anything but cylindrical; there was no step or wedge shape that I recall that would mechanically lift the control piston.




The assembly diagram seems to support that there is no mechanical connection to the control piston.


I would suggest that you are indeed correct and total failure of the hydraulic side of the brake system will also fail the trailer service line. The thinking error that made all of this much more complicated of a post than it needs to be is the simple fact that it is the action of the hydraulic MC that activates the air hydraulic brake booster (aka airpack).

Hydraulic fluid loss - no master cylinder function. No master cylinder function - no air pack function. No air pack function - no service air line function.


MSeriesRebuild explained the air assisted hydraulic brakes back in 2008:

"Well nobody covered exactly how the system functions in detail. When the pedal is applied, fluid pressure travels from the M/C to the booster. This activates the diapram in the hydraulic portion of the booster. When the diaphram opens the airway allowing compressed air from the storage tanks to flow, the piston inside the large barrel portion of the booster assembly is forced into motion by air pressure. An atmospheric line on the opposite side of the piston allows air on that side of the piston to escape. The air operated piston is forcing hydraulic or brake fluid out to the wheel cylinders, this applies the brakes. When the pedal is released, the hydraulic diaphram is closed by spring pressure, the line on the atmospheric side allows air from outside to flow in so the piston can return to the neutral position via spring pressure. Fluid flows backwards toward the M/C allowing brakes to release. The atmospheric line is plumbed into the air cleaner system on M35A2 trucks so the system draws & expels clean air. That's about as simple as I can explain it. "

 

[fleetmech]

cattlerepairman, thanks so much for the detailed reply! That's just what I was looking for and, unfortunately, I appear to be correct.

Knowing that I'm intensely paranoid of the single circuit brakes, a friend of mine suggested (half in jest) that I just fix up my m105 and tow it around everywhere as a secondary brake wagon. Not a real solution, obviously, but since I hadn't seen any other control systems on the pedal or master, and I do want to get the 105 all fixed up someday, I started looking around for what actually did the air application.

 

[cattlerepairman]

Fleetmech. it's one of these things I had never really thought about, so thanks for bringing it up! Turns out, at least the Deuce is consistent! If the hydraulic side fails, one's sphincter will grab the seat cushion really tightly. Imagine having this happen while pulling a 10,000 lbs air brake trailer. It better be on flat or gently rising ground, not in the mountains.

The M48 semi trailer variants (what's the other one...275?) at least have a trailer brake valve that is hand operated for direct service line brake control. They can use their semi trailer as a "brake wagon".

Maybe that is an upgrade for the cargo truck that you can make to use the M105 the same way (hopefully it's loaded, otherwise that single axle is just going to hop around unhappily).

 

[fleetmech]

Isn't it just like a Deuce to prove that even consistency can be worrisome? No redundancy on the truck or trailer brakes, small parking brake and no Jakes... just Soldier A and Soldier B having a quick discussion about whether its better to hit the mess hall or the generals car...

Ive been spending a lot of evenings researching parts and pieces and thinking over different ways to build a new braking system for the ol' soldier. I lost a classic car to a single circuit failure years ago, so it weighs on my mind a bit.

Not much in the way of mountains where I live, thankfully, but plenty of small towns and hilly backroads.

 

[m715mike]

Like this....

Adding a Trailer Brake Control Valve to an M35A2

I recently got a trailer brake control valve from an M931A2 and would like to put it on my M35A2 so I can manually apply trailer brakes. They also call these a Johnny Bar, Johnson Bar, etc. It had 3 lines coming out of the back and I was curious where they need to be routed to. One of the ports...

www.steelsoldiers.com

 

[rmesgt]

Here is the air line diagram:

View attachment 804931

So, what you are asking is "is the piston (4 and control valve piston 5) inside the compensator mounted to the airpack actuated by hydraulic pressure, air pressure or mechanically"?

View attachment 804936

Based on the assembly instructions, the control piston (5) protrudes into the bore of the slave cylinder. That would suggest that it is being activated by hydraulic pressure.

View attachment 804937

From rebuilding my air pack I do not remember the hydrualic piston inside the slave cylinder being anything but cylindrical; there was no step or wedge shape that I recall that would mechanically lift the control piston.


View attachment 804938

The assembly diagram seems to support that there is no mechanical connection to the control piston.


I would suggest that you are indeed correct and total failure of the hydraulic side of the brake system will also fail the trailer service line. The thinking error that made all of this much more complicated of a post than it needs to be is the simple fact that it is the action of the hydraulic MC that activates the air hydraulic brake booster (aka airpack).

Hydraulic fluid loss - no master cylinder function. No master cylinder function - no air pack function. No air pack function - no service air line function.


MSeriesRebuild explained the air assisted hydraulic brakes back in 2008:

"Well nobody covered exactly how the system functions in detail. When the pedal is applied, fluid pressure travels from the M/C to the booster. This activates the diapram in the hydraulic portion of the booster. When the diaphram opens the airway allowing compressed air from the storage tanks to flow, the piston inside the large barrel portion of the booster assembly is forced into motion by air pressure. An atmospheric line on the opposite side of the piston allows air on that side of the piston to escape. The air operated piston is forcing hydraulic or brake fluid out to the wheel cylinders, this applies the brakes. When the pedal is released, the hydraulic diaphram is closed by spring pressure, the line on the atmospheric side allows air from outside to flow in so the piston can return to the neutral position via spring pressure. Fluid flows backwards toward the M/C allowing brakes to release. The atmospheric line is plumbed into the air cleaner system on M35A2 trucks so the system draws & expels clean air. That's about as simple as I can explain it. "

Can you direct me to these diagrams? Which TM (or other manual) can a person find these images? TIA...

Leo

 

[HDN]

I read this post and was wondering how this works for dual-circuit systems:



Based on this diagram from page 1-4 of SMI 9-2320-209-14&P, both air packs run the trailer service brake connection, so the trailer will still get some kind of air signal if a brake circuit fails. I wonder, though, if the air signal is the same strength or weaker in a hydraulic brake circuit failure. I'm thinking it would be the same strength if the air pressure remains constant, which it should provided no air components fail at the same time as a hydraulic circuit.

 

[rmesgt]

My deuce seems to have a single system, as I have only one set of air tanks and only one air-hydraulic booster. I finally found the SMI 9-2320-209-14&P manual. As I was reading it, I noticed that it is missing page 1-2. I have found this manual in other places and they too are missing page 1-2. Apparently the military forgot to include it. Quite annoying. Thank you for your post. I am SLOWLY beginning to understand an M35A3 brake system...

 

[HDN]

My deuce seems to have a single system, as I have only one set of air tanks and only one air-hydraulic booster. I finally found the SMI 9-2320-209-14&P manual. As I was reading it, I noticed that it is missing page 1-2. I have found this manual in other places and they too are missing page 1-2. Apparently the military forgot to include it. Quite annoying. Thank you for your post. I am SLOWLY beginning to understand an M35A3 brake system...

Most deuces will have a single-circuit system. Some built in the 80s came from the factory with dual-circuit brakes. Maybe some in the military were retrofitted as evidenced by the document I referred to. All M44A3 trucks came with dual-circuit brakes from the factory.

The bottom line is that the trailer service air brakes are worked by the air pack(s). On a single-circuit system, if hydraulics are lost, all brakes, including trailer brakes, are lost because there's no hydraulics to work the air pack. On a dual-circuit system, there will still be trailer brakes, and I think they will still be full-strength as they should still be getting the same amount of air regardless whether only one air pack is working.