Deuce brakes

[m16ty]

I have to admit this is kind of a "pet peave" of mine .

Most people on here say a deuce has air-over-hyd brakes. Which is wrong and can be misleading. A deuce has air-boosted hyd brakes. It's basicly the same as most other hyd systems other than it uses air pressure instead of vaccume to boost the pressure. In a air-over-hyd system you have no brakes at all if you loose air pressure because there is no linkage between the m/c and the pedal, only air lines. I'm pretty sure air-over-hyd systems are illegal for on road use on vehicles.

A m105 trailer does have air-over-hyd brakes.

 

[Boatcarpenter]

Air-Over-Hydraulic Brake System
Seems as though there is a connection between the MC and the brake pedal.

 

[Angus1]

I always call them air assisted brakes.

 

[Jake0147]

You can find this described many places, and you will find many places (including technical references both old and new) where the term "air over hydraulic" is used incorrectly.

Deuce brakes are hydraulic brakes as far as any governing body is concerned. More specifically, they are power assisted hydraulic brakes. More specifically than that, they are air assisted hydraulic brakes. But they are hydraulic. The "air" need not be specified any more than the vacuum assist needs to be specified on your car, or the hydraulic assist needs to be specified on your diesel pickup truck.

First and foremost, let me say that these are way oversimplified and generalized....

With hydraulic brakes, the pedal is directly connected to a hydraulic cylinder (the master cylinder) and the force applied to the pedal is transferred by the fluid directly to the slave cylinders (wheel cylinders) where it is converted back to mechanical motion.
With assisted hydraulic brakes, this remains the same, however there is a power assist device. In most modern stuff, this device applies additional force to the mechanical linkage between the pedal and the master cylinder that is far more but directly proportional to your input force. More common in days gone by, and on deuce and a halfs in particular, an "inline" type booster is used, where the assist is added after the master cylinder and before the wheel cylinders.
With full air brakes, the treadle valve (brake pedal) operates a variable air pressure regulator. This pressurizes the air line to each wheel, where the air acts on a diaphragm, the diaphragm then operates a fully mechanical brake. Most commonly with a rotating "camshaft" that spreads the shoes as it rotates. Another common type is simply a wedge that is perpendicular to the direction of the brake shoes, and as the wedge is driven between them, the shoes expand.
With air over hydraulic brakes, you essentially have two complete braking systems. It starts as full air, in that you have a treadle valve in lieu of a brake pedal, when you step on the brakes you control only air. This part of the system is identical to full air brakes. The air then instead of branching to a diaphragm at each wheel, will simply act on a single diaphragm. That diaphragm is attached to a brake master cylinder of a completely independent hydraulic brake system. From that master cylinder, fluid pressure is applied to hydraulic wheel cylinders to activate the brakes. Virtually two full complete braking systems, with the first one serving only to activate the second one. This is not what is present on the deuce and a half. The M105 "deuce trailer" does use a true air over hydraulic braking system. You only hitch up an air line, the air runs the hydraulics.

 

[Jake0147]

I'm pretty sure air-over-hyd systems are illegal for on road use on vehicles.

No, not at all. They're kind of (IMHO) redundant and obsolete, but they are quite legal. We used to have several trucks at work that were so equipped. Somebody somewhere may have (and should if they have not , again IMHO) stopped them from new production, but there's bunches of them out there that are not illegal.

To my knowledge and understanding, air over hydraulic requires the endorsement or lack of restriction (depending on individual state law) that allows you to operate air brakes. Air assisted hydraulic brakes do not require any endorsement of any kind, any place or anywhere.

 

[Engine5]

You can call them blueberry cheesecake over hydraulic if you like, as long as they stop the truck. That's all I'm concerned about. Well that and blueberry cheesecake.

 

[army98]

The key should be "as long as they work"

 

[m16ty]

Jake, you explained it much better than me .

What kind of trucks had air-over-hyd? I've never seen a air-over-hyd system on a truck. I not doubting you, just curious. Only time I've run into air-over-hyd systems is on construction equipment and trailers.

 

[Boatcarpenter]

According to figure 3-47 in the link I posted, a Deuce would have air-over-hydraulic brakes.
Jake0147, I think you are a little off when you say in your explanation of an air over hydraulic brake system that "you essentially have two complete braking systems." All I see is one typical hydraulic brake system that happens to have the pressure applied to the fluid in the MC by air rather than applied by direct mechanical linkage to the brake pedal. To me this does not constitute your " two complete braking systems"
BC

 

[DieselBob]

Jake, you explained it much better than me .

What kind of trucks had air-over-hyd? I've never seen a air-over-hyd system on a truck. I not doubting you, just curious. Only time I've run into air-over-hyd systems is on construction equipment and trailers.

Back in the late 70"s early 80's when I was a truck mechanic the "R" and "DM" Mack's I worked on had "Air over hydraulic" front axle disc brakes. There was a air chamber that was plumbed into the standard air brake system through a proportioning valve and it operated a single master cylinder that applied the disc brakes. Worked very well.

 

[Jake0147]

According to figure 3-47 in the link I posted, a Deuce would have air-over-hydraulic brakes.
Jake0147, I think you are a little off when you say in your explanation of an air over hydraulic brake system that "you essentially have two complete braking systems." All I see is one typical hydraulic brake system that happens to have the pressure applied to the fluid in the MC by air rather than applied by direct mechanical linkage to the brake pedal. To me this does not constitute your " two complete braking systems"
BC

I believe that the use of the term "air over hydraulic" is not used correctly in the case of this particular reference manual. It's not uncommon to find both air over and air assist mislabeled in reference materials.
IF this information comes from the manufacturer, then this MAY make it technically correct to call it air over hydraulic for service purposes (but I don't think so). It would however be a "proper noun" when you said it that way. For comparative purposes, registration purposes, DMV purposes, and operator classification purposes, or describing it to a local mechanic in the hopes of getting advice for repair, the system described in the manual you linked to is not "air over hydraulic", it is "hydraulic".

Jake, you explained it much better than me .

What kind of trucks had air-over-hyd? I've never seen a air-over-hyd system on a truck. I not doubting you, just curious. Only time I've run into air-over-hyd systems is on construction equipment and trailers.

The ones we had were UDs in the class six trim. The Isuzu cabover's used it as well. I'm quite sure there are others, but these are the only one's I've put my own hands on.

 

[m16ty]

You don't call most passenger vehicles built today "vacuum over hyd". I've never read in a manual where they called a system "vacuum over hyd". The deuce system is basicly the same as a vacuum boosted system. Only difference is the deuce uses air pressure instead of vacuum to boost the pressure.

 

[Boatcarpenter]

OK Jake0147, pretend I'm from Missouri, The Show Me State, and show me some technical information that describes your version of a air-over-hydraulic brake system and labels it as such for comparative purposes. I'm trying to figure this whole thing out.
To the best of my knowledge, brake shoes are operated by the following means:, straight mechanical, hydraulic pressure, air pressure, or electrically. So in a strict sense, your'e right, the brakes in the diagram are hydraulic. The braking system as a complete system is more than just hydraulic though. There is an air component to the system that has to be considered when looking for advice or whatever to repair or maintain the system properly. Play semantics and call it what you want, air-over hydraulic, air/hydraulic, air and hydraulic, air-assist or the ever popular cheesecake system, there are two systems operating with different mediums combined into one to make a complete braking system. I consider that to be needed information when describing the system to someone.
Also, I found this reference to a patent relating to an air assisted braking system for a vehicle. Is this similar to what you mean by air assist?
Title:
[SIZE=+1]Air assisted braking system [/SIZE]
Document Type and Number:
United States Patent 4288126



Abstract:
This invention relates to an air powered braking system for a vehicle. The air pressure in the front and rear brake systems is regulated by a respective one of a pair of air pressure valves. The air valves are hydraulically controlled and each valve (16) and (26) is independently connected to a driver operable hydraulic master cylinder so that the air pressure in each of the front or rear systems is proportional to the hydraulic pressure generated at the master cylinder (11). A hydraulic reducing valve is interposed in the hydraulic circuit between the master cylinder and the rear brake system air valve and operation of the reducing valve reduces the hydraulic pressure to that air valve and hence reduces the air pressure to the rear brakes relative to the air pressure to the front brakes.
BC

 

[Jake0147]

I havn't had a good book collection since I was at Ryder, and my UD stuff went with the last UD to go down the road. I did find this right off, it's probably better for this purpose anyhow, showing "the whole picture" instead of component repairs and trouble trees.

http://www.gnb.ca/0276/vehicle/pdf/ab_manual-e.pdf

Page 22 of the manual or 23 of the PDF has descriptions. For the purposes of this post, you will not find a deuce brake system since the single outlet master is obsolete, the one they show is simply a dual hydraulic outlet to dual air packs. It does shows very clearly (I think...) the difference between the air being used as a primary system and air being used as a secondary system to assist the primary system.

And if you didn't catch it, it's Canadian, so this will not affect any US or state laws regarding the operation of air brake equipped vehicles.

That abstract from the patent thing didn't really make sense, so after reading the whole thing (google United States Patent 4288126) that really does not sound like an automotive braking system I am familiar with. I see no reason it couldn't work out, but I can't envision the purpose. From my understanding of what that inventor is trying to say, This would be "hydraulic over air over hydraulic". Three primary systems. In this case the first hydraulic system is a primary braking system, it doesn't "assist" the air brakes, but rather it "activates" them. The air brakes in turn are a primary system, since they dont' "assist" the second set of hydraulics, but rather they activate them. and individual hydraulic systems at the wheels are primary hydraulic systems because they don't "assist" the linings, rather they activate them.

 

[m16ty]

Air-over-hyd systems could be called air-activated-hyd. You remove the air pressure and the brakes won't work at all because there is no air to activate them. In a air-assist-hyd system (as in the deuce) the brakes will still work but will be hard to apply enough pressure without the air. The main advantage of a air-assist system over a air-over-hyd system is that if for some reason the air pressure fails you still have some braking ability.

As a side note, Some trailers are comming out with electric-over-hyd brakes on them. The tow vehicle uses a electric brake box that activates a coil connected to a m/c on the trailer to apply the trailer's hyd brakes. It's a step above straight electric brakes but still has some of the problems associated with electric brakes.

 

[Jake0147]

That is exactly what "over" means when it's used in this or similar context... "Over" means "activated" or "controlled".

I have not seen electric over hydraulic first hand as of yet, but I am looking forward to it... I think it'll be a step forward in the landscaping to light equipment trailer range. I'm fine with the electric part (after you ditch the POS hack job butcher that most factorys call "trailer wiring") It's the electro-mechanical part in the drums that we have a hard time with. Of course some of the cowboys we send that stuff out with could tear up an anvil, but it really sounds to me like the best of both worlds. Well, until you get into air anyhow.

 

[jesse poundstone]

thank you thank you thank you as a heavy equip mechanic im no stranger to air over hyd. brakes maybe the others will under stand the huge difference now.

 

[greenjeepster]

We also don't call air brakes "air over mechanical spring brakes"

I agree that it really doesn't matter what they are called as long as they work.

 

[m-35tom]

to a great extent you are all mostly right. it's all in how you look at it and how you understand it. in a deuce with air pressure up, the primary master cyl does NOT apply the brakes, it only controls the pressure regulator to the air pack. so in this instance it could be called air over hyd. now if the air pressure goes away completely, then and only then does the pressure from the primary MC push on the piston in the air pack MC to apply the brakes, just a back up system. so really you can call it either and be correct........

tom

 

[joeypushjr1]

wow easy guys lol.. interesting read. to me there just breaks . i know i still have breaks if i dont have air in the system. ive tried it on pourpos to see if i still have them in the even air went away.. but hey i learned a little something here.