dd3 dual circuit

[junkman42]

Has anyone ever seen a coach with a dual circuit brake system using DD3 chambers?  Is there anyone that has modified a single circuit system to dual circuit?  All thoughts appreciated no lawyer quotes I understand all of the ramifications.  Regards John L

[bevans6]

Can you explain what you mean by dual circuit, exactly?  What comes to mind is a typical spring brake dual circuit system with front and rear tanks, and a two way check valve to tie them together if one has issues.

Brian

[wildbob24]

My 1975 GMC Buffalo has a dual system with DD3s. 1975 was the first year that dual systems were required by the Feds, so any DD3 equipped bus (i.e. P8m4905s & Mc9s and others) built after that will have a dual system.

Bob

[bevans6]

Not exactly so.  MCI's for sure had a separate emergency brake system, with separate tank operating separate diaphragms in the DD-3 chambers and this is the normal way that MCI systems operated.  They did not have dual service brake systems - in normal operation the dry tank system operated both the front and rear brakes, and in emergency operation only the rear brakes were operated.  Dual brake systems as are typical with spring brakes have a front tank and a separate rear tank, front and rear brakes are completely separate in normal operation, and that is what is normally called a dual circuit braking system.

Brian

[junkman42]

Brian, the reason for My question is I would like to change the treadle valve and have just like You describe front and rear circuits with separate air tanks!  Perhaps not possible, thought someone had already been there before!  I have for many years converted hotrods/classic cars to dual circuit brakes as one of the first mods!  Thanks for the input.  John L

[bevans6]

The treadle valve is actually the same, or can be.  The reason spring brake systems can and do use a dual tank system for front and rear is that they actually have an independent third system that uses no air at all and is automatic - the spring brakes, which apply when air pressure falls to below 60 psi.  The treadle valve has two separate "channels", one for the front brakes and one for the rear brakes.  DD3 systems don't have the spring brake as an automatic fail-safe, but they still have two separate systems - the service system and the emergency system.  The treadle valve uses one "channel" for the service brake system which operates both front and rear brakes at the same time using air from the dry tank (also called the service tank).  The other "channel" (I don't know what else to call them) on the treadle valve is connected to the emergency tank and to the parking brake chamber on the DD-3 but not to the front brakes.  There is a shuttle valve connected to both the service brake and the emergency brake systems and if the shuttle valve detects that the service brake air pressure is lower than the emergency brake air pressure it switches, and the treadle valve will now operate the parking brake diaphragms on the rear brakes rather than the service brake diaphragms.  Since the emergency brake system is separated from the service brake system by a check valve, it retains the highest pressure that it ever sees, usually 120 psi, and is large enough to give several controlled applications to the parking brake chamber, which is 80% the size of the main chamber in the DD-3, so you get to actually drive the bus to a stop rather than have the spring brakes slowly (or suddenly) apply only up to 60% of normal braking force as you lose air pressure, and you have no control over what they are doing.

I'm not sure how you would go about changing a DD-3 system to a front/rear split system.  The treadle valve only has two channels, and both are already in use.  If you go to a front/rear split you will lose the ability to have the DD3 emergency brake system unless you have a three channel treadle valve (never heard of one, but never looked for one either).  If you used the existing second tank for your two tank system you would lose the parking brake as well as the emergency brake so you would have to add a third tank for the front/rear split to work, so you could set it up to retain the parking brake, which could be used as an emergency brake with the push/pull valve, but you won't be DOT legal any longer since the push pull valve on a spring brake system has to pop automatically at 60 psi, and since your push/pull valve will retain 120 psi in the emergency tank it won't pop automatically.  The push-pull valve in a DD-3 system does pop automatically when air pressure in the emergency system falls to around 40 psi or so, so you do have an automatic element as well as the driver-controlled element in the DD-3 emergency system.

Now, I personally think the normal DD-3 system is better in many respects than a spring brake system, I wouldn't bother to change.  You already have the better system, switching to a dual front rear system seems like an upgrade but really isn't (hot rods aren't buses, after all, and I've upgraded my share of old british cars to dual brakes so I know where you are coming from on that one).

BTW, what bus do you have?  That makes a difference, as noted above after 1975 brake systems were upgraded across the board by government regulation.  Some earlier DD-3 systems probably don't work exactly as I have described.

Brian

[junkman42]

Brian, thanks for the input.  I would just feel more secure with a dual circuit or split system!  I am not overly impressed with My mc7's braking ability!  My system is in good order but I have never slid a tire in a panic stop.  Anyhow thanks very much.  By the way My coach is a 71 manufacture year titled 72!  Regards John L.

[bevans6]

Changing to a dual circuit system won't improve or change the braking performance of your bus at all, and it will make it arguably less reliable.  You're not supposed to be able to lock up tires on a bus.  If you want stronger brake performance you can put new drums on with perfect friction surface and diameter-matched shoes with more aggressive material that will give you more grab.  You might also install larger front chambers, but that might start to lock the front tires in a panic stop, and that might not be good.  Anyway, your bus, your choice, have fun with it.

Brian

[luvrbus]

The older Eagles in 1975 with DD3 brakes just used a E-6 foot valve and a check valve to pass the requirement for the DOT dual air system fwiw