Park Brakes - DD3 Brake Chambers

MCI coaches, with the exception of the Renaissance coach, all use DD3 brake chambers on the drive axle for the Park Brakes. When the park brakes are applied, air pressure is routed to the rear of the chamber that applies the brakes. At the front of the chamber are rolling "locks" which wedge themselves between the shaft and the body of the chamber. If the air were to leak off of the park brake diaphragm, these locks will hold the brakes in the applied position. To release these locks, you follow the steps listed below:

  1. Start the coach and allow the air pressure to build to 120 pounds.
  2. Release the park brakes by pressing down the park brake push/pull valve in the driver's area.
  3. Firmly apply the Service Brakes. This causes the brake chambers to apply further than the park brakes had applied, and this releases the rolling "locks" from the shafts.

Two problems that have been encountered are listed below. Also listed will be the troubleshooting procedure for fixing it.

  1. Park brakes will not release.
    • Usually having too much air pressure applying the park brakes causes this. In the air supply line for the park brakes is a pressure regulator which is set to 85 pounds. This is the amount of air pressure used to apply the park brakes, not the 120-pound system pressure. If this regulator is set too high, or the regulator has been removed, the brakes will be applied too firmly to be released by the service brake application. Setting this regulator too low will affect the stopping/holding capabilities of the park brake. 85# is the magic number to remember.
  2. Air is leaking out of the R-14 relay valve located above the drive axle (On ABS equipped coaches, air leaks out of the modulator valve.)
    • In the DD3 brake chamber, the park brake diaphragm separates the park brake section and the service brake section. If this diaphragm develops a hole in it, air will leak into the service brake section. When the service brakes are not applied, this section is vented to the exhaust port on the relay valve (With ABS, the exhaust port on the modulator valve.) Any air entering the service brake chamber will be exhausted out immediately thus causing the illusion of a faulty relay valve (With ABS, the modulator valve.)
    • To troubleshoot this situation, remove the service brake airline from one of the DD3 brake chambers. (This is the middle hose on the chamber) If air is leaking out of the service brake port, that is the chamber with the leaking park brake diaphragm. If air is still leaking out of the relay valve and the hose in your hand, then it is the chamber on the opposite side of the axle (On coaches with ABS, air leaking out of the RH modulator valve would mean the leaking park brake diaphragm will be in the RH DD3 brake chamber, LH modulator valve would point to a leak in the LH side.)

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