drjconley

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

73m819

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

__________________
Ron
Gainesville, Ga.
vietnam-1/66-8/68
42 5t? autocar tractor, gone
43m20, 12t, converted to tractor, gone
52m52, tractor, gone
73m819, tractor/wrecker AKA Wrecked Wrecker
77xm977 10t 8x8, to be a xm 983 sold
What ever else that can be drug home
Use CUCVs, a small step below BOBBERS as wheel chocks
GOD help the MV world if i win the LOTTERY
LONG LIVE BIG GREEN IRON

gimpyrobb

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.

__________________
http://www.webn.com/player/?station=...l&mid=22097950

saddamsnightmare

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

Army Ed

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

m16ty

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.

MSeriesRebuild

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.

__________________
Charles Talbert
www.mseriesrebuild.com

steelsoldiers

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.

__________________
Chris Stansbury, founder of Chris' CUCV Homepage in 1998 and Steel Soldiers in 1999.

1985 AMG M998 HMMWV w/ Cummins 6BT

Father, lead me, 'cause I can't do this alone.

Unforgiven

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.

m16ty

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.