DD3 Brakes In-depth
If you want to achieve true brake mastery, namely: the function of airflow to the chambers. Below is a simplified air schematic that shows the drive axle brake chambers and related parts.
Shown in this schematic are the lines and components that are involved with just the drive axle brakes. The front and tag axle brakes are not included for simplicity’s sake. The front brake air lines referenced (in red) are involved with the drive axle brakes only in the event of either a loss of primary air pressure or failure of the foot valve. This will be explained in more detail shortly. Let’s start with the basic functions first.
Applying the park brakes involves supplying air to the rear port of the DD-3 brake chambers. This air pressure originates in the park brake reservoir, located in the drive axle area of D models. That air goes through a pressure regulator and into the top port of the inversion valve that’s mounted above the rear axle. This 85 psi air passes through the inversion valve, through the double check (DC) valve that’s attached to the inversion valve, and to the rear port of the DD-3 brake chamber. There shouldn’t be any air pressure on the other two ports of the brake chamber.
To release the parking brakes, the park brake valve (PP-1) is pressed in. If the button won’t stay in, the air pressure in the park brake tank is too low. Allow the coach to finish building air pressure.
When the PP-1 is pressed in, air pressure from the park brake valve flows to the Control (“C”) port on the inversion valve. This pressure disconnects the supply and delivery ports of the inversion valve, and the air pressure in the rear portion of the DD-3 chambers escapes out the exhaust port of the inversion valve.
The air pressure from the PP-1 also flows to the front port of the DD-3 chambers to release the manual locks. Remember that a firm, full-service brake application is required to completely release the manual locks.
A bit of trivia: Remember: 85 psi is used to apply the park brakes, but system air pressure is used to release the manual locks.
Comparing this air schematic with the previous one, the park brake circuit is identical to the previous drawing. The park brake valve is pressed in, and the air is routed to the inversion valve and the front port of the DD-3s. Air must always remain on the front port of the DD-3 chambers whenever the park brakes are released.
When the foot valve is pressed, air flows to the R12 relay valve in the rear of the coach, above the drive axle, and air pressure is applied to the center port of the DD-3 brake chambers. But you may be asking yourself, “Why is there a red airline to the control port on the R12 valve?” For basic brake operation, it does nothing. More details soon.
Another item to remember. Never have the park brakes and service brakes applied at the same time; double pressure would be applied to the brakes and damage will occur to the drive axle brakes. The airline plumbing in the coach air system is designed to prevent this from happening. There is an airline that is connected to the delivery side of the primary air circuit at the foot valve and is routed to the exhaust port of the park brake valve. If the park brakes are applied (PP-1 button is up), air pressure from the foot valve goes into the exhaust port of the PP-1, out the delivery port and releases the park brakes the same way as pressing the park brake button in.
A troubleshooting tip: if air is leaking out of the exhaust port of the foot valve when no brake application is being made, it’s usually a problem with the park brake (PP-1) valve, since the exhaust port of that valve is connected to the delivery port of the foot valve. The delivery ports and exhaust port of the foot valve are connected when there’s no service brake application.
In this example, the primary brake reservoir is fully pressurized, but the foot valve has failed to apply air pressure to the primary brakes. This is a serious scenario since the drive brakes provide the majority of stopping power for the coach. As a sort of insurance policy, the red airline (mentioned earlier) from the front brake circuit gets utilized. It provides air pressure to the R12 relay valve above the drive axle to apply the drive (and tag) axle brakes. Even though the brakes are being applied, there is a bit of hesitation as the brake pedal is being pressed, so an experienced driver should notice the braking system has an issue and needs servicing immediately.
This is a dangerous scenario. There are numerous reminders, both in manuals and a warning decal on the dash, not to operate a coach without adequate air pressure. But in the event of an air pressure failure, while driving, the coach air system has a safety contingency in place. Located in the spare tire compartment is a second inversion valve, officially called the emergency standby valve.
Using any of the previous schematics, you’ll see a green airline originating from the primary air tank and routing down to the control port on the side of the emergency standby valve. As long as air pressure is present at this port, air pressure from the supply port on the top of the valve is isolated from the delivery port.
When the primary brake tank loses its air pressure, the pressure is also removed from the control port; the two airlines shown above are connected, and secondary (front) brake pressure is routed to the other end of the double-check (DC) valve located above the drive axle. This air pressure continues to the rear port of the DD-3 chambers and is used to apply the parking brakes, in place of the missing service brakes. In this situation, the tag brakes won’t function, and the brakes will have a different “feel” to them. Again, an experienced driver should notice the change in braking and should have the coach inspected immediately.