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Articles
NEWS | Sept. 7, 2022

HEMTT: Parking Brake Problems? Check Exhaust Port

BLUF:  Keep exhaust port on the parking brake valve exhaust hose free of debris.
 

Dear Editor,

I’m a LAR and recently worked on several HEMTTs that were having the same issue. The parking brakes were spongy and wouldn’t stay applied or, when applied, they’d release as the vehicle’s air systems reached about 90 psi.

Inoperable parking brakes render the vehicle not mission capable (NMC) and are a safety concern because the vehicle could potentially start moving on its own and damage equipment or injure personnel.

The problem was found to be with the parking brake valve exhaust hose, NSN 4720-01-287-9313, which runs from the inside of the HEMTT cab and through the frame and ends under the vehicle, between axles three (3) and four (4). The hose is orange and is shown here.
 
Parking brake valve exhaust hose
Parking brake valve exhaust hose
 
At the very end of the hose is an exhaust port (opening) which allows the parking brake valve in the cab to properly exhaust air. Because the exhaust port is under the vehicle, it’s often overlooked and dirt and debris can clog the port. When the exhaust port is clogged it can’t properly exhaust air and the parking brake won’t work.
 
 Ensure exhaust port isn’t clogged
Ensure exhaust port isn’t clogged
 
The problem is hard to troubleshoot, can consume four (4) to eight (8) hours of work and isn’t covered in TM 9-2320-326-13&P (Sep 20). But there’s an easy solution.

Just routinely check the exhaust port and remove any dirt and debris that could potentially clog the port. You’ll know that the exhaust port is clear of debris when you can clearly hear air venting from the exhaust hose as the parking brakes are released.
 
Oh, and the estimated cost savings can be anywhere from $860 to $6100 on parts alone.
 
Corey Martinez
Ft Bliss, TX
Editor’s note: Thanks, Mr. Martinez! That’s a great tip for HEMTT users, and they’ll want to jot this info down until the TM is updated.
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