Parking Brake Problem

[eagle19952]

http://www.bendixvrc.com/itemDisplay.asp?documentID=4420

EVERY 3 MONTHS, 900 OPERATING HOURS, OR 25,000
MILES
When grease fitting is provided, grease with Lubriplate
"Aero" lubricant.

[Tikvah]

There has been some discussion among friends as to the identification of my brake chambers.  Below are a few pictures.

[bevans6]

Those look like DD-3's to me.  MCI used nothing else until they stopped using them sometime in the 1990's I think.  If you set the parking brake with the push/pull valve you get (if the regulator is working) an 80 - 85 psi application to the parking brake chamber, which is the rear-most chamber.  That expands the parking brake diaphragm and pushes out the push rod while at the same time removing pressure from the locking section so the locks engage holding the push rod out.  If you then do a service brake application it puts air pressure into the service section (middle hose) and what you get is the average of the service brake pressure and the parking brake pressure based on the relative volumes of each chamber.  It might push the push-rod a tiny bit harder, but it's not the same as a full normal service brake application pressure.  DD-3's don't compound the parking and service brake pressure the way that spring brakes do.  Regardless it is poor practice to "set" the parking brake with a service brake application after the parking brake is applied (but I think we all do at some point).

As far as partial releases go, as soon as air is applied to the locking port the rollers are released.  If the pushrod hasn't relaxed due to air leaking out from the parking brake chamber the rod will just unlock and the brakes will release without a service brake application. That's because the locks don't actually lock onto the push-rod until it relaxes and moves back in to the chamber a tiny bit.  Normally a brake application is required.  The design is such that the pushrod pretty much will either lock or unlock, and being sticky won't cause a partial unlock or make the brakes drag.  If it didn't they would drag all the time.  There is a pretty big spring inside the DD-3 to force the pushrod back in, plus the shoe springs inside the brakes.  You do need to sparingly lube the locking section of the DD-3, but I don't look for it to cause a partial release of the parking brake unless the can is about shot.

Brian