Brakes --why air?

[dirtyfingernails]

I have a question that may be a dumb. If the deuce brakes are hydraulic, why do you HAVE to rely on the air assist to stop? In other words, can you stop the truck with the air system broken? Thanks.
Steve

 

[FreightTrain]

You can stop the truck if you have legs like Arnold but without it you are the one building all the pressure.The air pack reads your foot pressure and boost the pressures WAY up.There is no way you could even come close to the pressures the air pack makes.In a car your only pushing 2 calipers and a wheel cylinder(for most Disk front drum rear setups) but on a deuce your pushing 6 VERY fluid hungry wheel cylinders.Double pistons each.Also,The reason we have air assist instead of Vaccumn is diesels don't have ANY Vaccumn.They would have to add a separate pump and still have the air system for trailer brakes.

 

[FSBruva]

Cranetruck did a test once...

Standby while I use the Search function to find the post.

Matt

 

[FSBruva]

http://steelsoldiers.com/index.php?name=PNphpBB2&file=viewtopic&t=3167&highlight=brake+psi

Matt

 

[steelandcanvas]

There are no dumb questions, maybe it has been asked before, but not dumb. The guys are here on this forum to help all of us enjoy this addiction of working on MV's. Stay here long enough and you'll be answering questions asked by new people. Don't be concerned about looking dumb, I'm a recent new member also!

 

[steelandcanvas]

Clock is ticking Matt!

 

[Barrman]

It isn't just a diesel type thing either. The Gasser M35's have the same air assist brake system. The diesel replaced them, so it was designed into the truck from the start.

By the way, you can stop the truck without it. I have resurected my truck from sitting in a field for the last 25 years. I got it running first, then made the hydraulic part of the brakes work. I haven't messed with the air part of the brakes to make them work yet. It stops fine as long as you push really,really hard. I also haven't had the truck over 25 mph yet and don't want to until it is all working.

 

[Autocar]

Can anyone tell me why hydraulic brakes are used? I haven't been able to figure it out. You have everything needed for air brakes already(compressor, air tanks, trailer glad hands, etc.) so why the second brake system? Surely a treadle valve and spring brakes must be cheaper than air/hydraulic booster and pinion e-brake, without the additional complexity?

 

[Barrman]

What were over the road semi trucks using in 1949 and 1950? State of the art for 1950 sure does seem like the stone age to us doesn't it?

 

[houdel]

I have a question that may be a dumb. If the deuce brakes are hydraulic, why do you HAVE to rely on the air assist to stop? In other words, can you stop the truck with the air system broken? Thanks. Steve

Not a dumb question. For whatever reason, the military decided to go with a hydraulic brake system at the time. Remember, these trucks were designed in 1949 and went into production in 1950, so we are talking 1950s technology. Maybe at the time the military was not comfortable with air brake technology in 1950.

All the air system does is provide a power assist to the braking system, it will still function without air pressure, although as mentioned you need muscles like Arnold S. to work the brakes without the air assist. The few vehicles of the time with "power" brakes (all gas engines) used engine provided vacuum assisted brakes.

Since a diesel does not develop manifold vacuum like a gasser, they needed either a vacuum pump or an air compressor to provide brake assist. The military, in its infinite wisdom, decided to go with air assist. Most later medium and HD vehicles also used the air assisted brakes. I suppose there were at least two reasons to continue with the air assisted brakes, one was to simplify mechanic training, the second that in event of air supply failure, the truck would still have braking ability, albeit with significantly more pedal effort on the part of the operator. In the case of a tactical vehicle, the ability to operate the vehicle, even with some systems failure, is paramount. The air assisted hydraulic brakes provided the military with a "second chance" braking system, to improve tactical reliability.

 

[m139h2otruck]

I remember reading somewhere (maybe on this site) that the air assist/hydro brakes were speced for their submerge-ability compared to the 1950's state of the art air brakes.

Don't believe everything you see or read on the Web!!

 

[dirtyfingernails]

Thank you all. The reason I asked was the TM 9-2320-361-10 says that the air reservoirs provide enough reserve air to stop the vehicle in the event of a failure of the air supply. You've clarified this, somewhat. So, if you did need to stop the thing without air assist, it would be possible? or is it impossible to generate the force necessary without air? Sounds like the latter based on your comments and the TM. Since this is a single stage (or reservoir) master cylinder, should I replace it with new as well as the brake cylinders and the booster immediately to ensure that it is safe? I will definitely go through it all. I've got to get up to speed on bleeding this thing--looks like the booster complicates this process.

 

[houdel]

A. You can stop the deuce without air assist - it just takes a lot of foot power. Per Bjorn's data, with air assist you can get ~1800 psi in the brake system vs ~400 psi with foot power alone. I regularly drive my Deuce around the yard sans air assist while waiting for the air pressure to build up and the brakes work fine, but I am talking 3-4 mph, not 40-50 mph.

B. I wouldn't worry about replacing the M/C or wheel cylinders unless they are leaking or otherwise defective. Same for the AHU (air hydraulic unit), if I ain't broke, don't screw with it (good philosophy for most any part of the Deuce, routine maintenance excepted). The only thing I WOULD suggest is to pull the air pipe off the back of the AHU and give it couple of squirts of air tool oil or similar very light oil into the air chamber, that helps keep the AHU working freely (I use Breakfree CLP, for what it is worth).

C. The AHU (brake booster/power assist) doesn't really complicate the brake bleeding process. There is a bleeder on the AHU, bleed that first, then bleed the wheel cylinders, starting with the furthest from the M/C and work your way in. Just make sure to use the same fluid for make up as is in your system - USUALLY DOT 5 (BFS, or Brake Fluid, Silicone, in military nomenclature), although some do have DOT 3, the regular civilian type brake fluid. If in doubt, take a sample from your master cylinder, put it in a small jar, add some water, shake, then let it set for a while. If the water separates out, your system has BFS. If the water mixes with the brake fluid you have DOT 3 in your system. Either one will work fine, just don't mix the two. If you need to purchase some silicone brake fluid, make sure it says "SILICONE" on the label. There are some brake fluids out there labeled DOT 5.1 or DOT 5+ which are NOT silicone based and should NOT be mixed with silicone fluid. If you search this forum for "power brake bleeder" or similar wording you will find a couple of threads describing a home made power brake bleeder based on a small garden pump-up insecticide sprayer, which make the bleeding process about 1000 times easier. Well worth the effort to build one if you intend to do a complete bleed. You can get surplus BFS for about $30/gallon, a lot better price than $10/pint at your local auto parts store.

 

[acetomatoco]

I will have some BFS at the VT show next weekend... $30 per gallon...genuine gummint stuff...purple...

 

[acetomatoco]

Regarding why air? How would you fill and change tires, paint in the field, blow up your air mattress, charge the M2 burners for your kitchen, and a million other things compressed air is used for...like running the windshield wipers...

 

[cranetruck]

FWIW, the M656/XM757 8x8 trucks designed in the mid 60's, use 100% air for braking. One more air tank was added and check valves between the tanks keep pressure up in the event of upstream air loss....

 

[dirtyfingernails]

Thanks again everyone. Is all DOT 5 going to be purple? What is breakfree CLP and where can you buy it?
Steve

 

[doghead]

Dot5 is usually purple when its new(turns yellowish or even clear with time). CLP is a cleaner, lube, protectant. I bought it at Walmart in the sporting goods dept. It is near the gun cleaning stuff. (aerosol spray)

 

[pfarber]

Hydraulics to the cylinders is best because fluids are incompressible, and transfer 100% (theoretically) of the force to the other end. The air part also assists not only with pressure, but with the volume of fluid needed to move all 6 slaves.

My CCKW generates about 1000psi to the slaves with nothing moer than vacuum (its a gas engine). There really isn't any new technology in the M35 vs 20+ year old tech at the time (circa 1960's).

 

[houdel]

You can get Breakfree CLP at Wally World, sometimes it is in sporting goods, sometimes automotive, or you can often find it at gun/sporting goods shops and hardware stores. IMHO, it is the greatest stuff since sliced bread. It removes light rust, lubricates and protects against further rusting. It dries so the surface is not oily but still prevents rust. I use it on my table saw which I have to leave outdoors in a carport, wiped it down with CLP and it hasn't rusted in two years. If you are long enough in the tooth, you will recall when the M-16 rifle was first introduced in Vietnam they had a lot of problems with the gun jamming. CLP was the only thing which would keep the guns working until the military made some mods to improve reliability.