air over hydraulic brakes need explanation

i understand hydraulic brakes, i understand air brakes, but i dont get air over hydraulic. why didnt the manufacturers just go with air brakes, it seems like there are two systems when one would work. also does the air pressure release the brakes like a air system, or does air pressure apply the brakes like a hydraulic system. if the hydraulic system has no fluid would the tires lock up like air or not work like hydraulic. sorry for my ignorance with this issue.
jim

no fluid, no brakes, the air acts as a booster to the hyds .to give it power brakes. these trucks were designed in the late 40s/ early 50s. this was a standard brake system for medium trucks back then

Think about car brakes. The vacume acts as a power assist to redue the force you have to apply on the peddle. Well our trucks are diesel(basicly). Diesel makes no vacume, so they have an air compressor. The air pressure acts as the assist to reduce peddle pressure.

December 22, 2008.

Dear DRJCONLEY:

Take a look at the -20 and -30 manuals for the M35A2 to see the systems set up. The Army was, for 1948, eminently logical on the deuce's brake system, inasuch as they needed power assistance to make them work, they needed to be able to move the truck quickly in emergencies without having the air build up, and they had to be able to tow them quickly also in an emergency.
If they were straight standard air brakes, any damage to the air system or time delay in building up the air, might concieveable in some circumstances, loose the truck and the load.
EG: Enemy attack shoots off an air line, or enemy attacks motor pool or convoy when the trucks have been shut down, brakes are locked, truck's immoveable, or unstoppable, depending on what happened..... The hydraulics work the brakes from the pedal and master valve, the airpack senses pedal inputs and assists in pushing the master cylinder to the apply position. The airpack serves the same purpose as the vacuum assist on most other modern car and truck brake systems.
The only weakness in an unmodified (to ESP standard M35A2) is the loss of any hydraulic line is gonna render the truck almost uncontrollable, hence the splitting of the hydraulics in the rebuilt M35's in the 1980's and almost all AF M35A2's.....
Basically, if the M35's working right, it takes less effort to stop it on my part as the driver, then my 1963 Swiss Unimog, which is much smaller in size, but with no assist to the hydraulic system..... A Unimog S404.114 in heavy traffic is a tiresome beast to have to stop, a Deuce is a tiresome beast to have to shift.....! Due to the sensitivity to failure of the hydraulic systems on the M35A2, it is essential that they be inspected and maintained to a higher standard then other vehicles in terms of preventative maintenance, as the parking brake WILL NOT STOP the truck, and neither will most telephone poles or trees if the truck has any momentum built up. I hope this will help you visualize the operation of the brakes, as they are different then any other truck's before or since.

Have a Safe and Happy Holiday Season,

Cheers,

Kyle F. McGrogan

personally from a military mechanic point of view i prefer the air over hydrolic.Its easier to make a recovery on,fix in a tactical situation where if you have an air issue you can still drive on.Where as just air brakes if there is a air issue your semi stuck with out having to cauge the brakes and using the spring brake over ride

The brakes work like everyone has stated but it is wrong to call them "air over hydraulic". They are "air assisted (or boosted) hydraulic". A M105 trailer is air over hydraulic and some construction equip. is but none of the trucks are. In a air over hydraulic system if you loose air pressure you have no brakes because the air activates the hyd. brakes. I think air over hyd. brakes are illegal in most states for on road use.

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.

MSeriesRebuild said:

That's about as simple as I can explain it.

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Fantastic explanation! It's an honor to have you as a member here, Charles. Welcome to the family. I know you will be a great contributor to the site.

MSeriesRebuild said:

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.

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This is what I was looking for. Thanks to whoever wrote this a couple of years ago. So basically, it's not even a direct hydraulic system. It's hydraulic from master cylinder to open the diaphram. Then air pressure from the air tank pushes a larger piston that in turn pushes a second stage of hydraulic fluid to the wheel cylinders. So a proportioning valve for a dual master cylinder would be meaningless. All the MC is doing is opening the diaphram for pressurized air and is not directly pushing the wheel cylinders (or very minimally). It's the air that's doing all of the work. So if either the first or second stage of the hydraulics fails, there are no brakes. And if the air supply fails there are no brakes. Now I see why the hydraulic power steering guy did what he did.

Unforgiven said:

And if the air supply fails there are no brakes. Now I see why the hydraulic power steering guy did what he did.

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This is not correct. If the air system fails you will still have brakes. They just won't have any boost from the booster. The pedal will be much harder to push but you will still have brakes. When no air is available the fluid from the MC flows through the booster and to the wheel cylinders.

I don't have any Organization or Depot manuals- anyone have a cutaway drawing(s) of the master cylinder and booster?? Training illustrations?

-Ken J

Another good source of brake system components and theories:

http://www.mpi.mb.ca/PDFs/AirBrakeManual/ABM_Section2.pdf

-Ken J

Sorry to ask what might be a neophyte question, but with an air assisted hydraulic system such as in my M54 if the truck sits and is not running will the brake pedal be stiff or soft? Right now I have not even had the chance to get my truck started (because I am working out of town this past month and have had no time to even go back to where it is parked) yet when I was up there seeing it for the first time, I remember stepping on the pedal and it being very soft. I figured at the time I needed to have air pressure up to have brake pedal and now I am perplexed and worried that my truck might not have working brakes? If I have no working brakes, what is the most common failure point to check? I assume MC and I hope not something bigger and more problematic to fix?

The brake pedal should be stiffer with no air pressure. If the pedal is soft you have issues on the hyd side. Could just need a bleeding from sitting or you could have a leak somewhere.

well, I hope it is only a master cylinder without enough fluid but if that is the case then it does have to be leaking out somewhere otherwise a bad MC I guess? No way to know until I get back up to the truck in August. I think I'll order a rebuild kit for the MC in the meantime.

First thing ADJUST the brakes, that WILL take some of the "soft" out, if still to soft, bleed, my 819 sat TWO years, engine could not be started, my brakes NEVER changed from the good pedel it had when it got shut down.

I would wait on the mc kit, if there is a leak, it will be most likely a wheel cylinder, my 52 m52 had a front wheel cylinder that leaked ONLY if the truck sat for more then a week, on day to day use, not a drop leaked

THANK YOU for doing some searching and finding this thread, this way others can see your problem and the fix, instead of another thread, the more threads spread out, the harder it is to search a answer

M16ty, now they have an air tank and emergency air supply with a breakaway device, putting the emergency air to actuate the brakes (or in normal operation, the trucks primary system). They are still popular I believe on commercial trailers with hydraulic brakes being pulled with an air system, like my 923, but do require some new equipment.

I know I am probably worried about nothing but what are the chances that it is the "brake/hydraulic assist" unit? Do they have many failure points like seals that go bad from sitting? Supposedly my truck was driven only 4 months ago but now I am not so sure about that. It could well be that it has been sitting over a year.

If you have the money, replace it with new or totally reconditioned. Internal rubber diaphragm elements dry rot. Water is your enemy where rust has eaten metal away. You'll find out when you have it apart...if there are elements that need replaced. In the case of air over hydraulic on a trailer, I see that was not the topic. Find new if it's available. My $02.

Someone mentioned "lubing" the Air over unit. Been looking thru some TM's and can't seem to find a reference. Anyone know what this is referring to and /or how to do this? Mine is acting up and will make the pedal like a rock stiff once pressure builds up. I am not driving on the road and am actively working on them. Any info pictures diagrams would be great as far as this "lubing" process goes.