How to replace Norgren AMU Pressure Protection Valves

General Information
On some AMU (Air Management Unit) pressure protection valves manufactured by Norgren, the internal O-ring may jump out of its groove, allowing the air to blow by the O-ring. This results in an air leak, with virtually no air flow from the secondary air supply to the AMU modules downstream of the pressure protection valves (air flow reduced by 99 percent).

When this condition exists, the secondary air gauge will display a drop in pressure and the air compressor will recycle more often to compensate for the air loss. Any air leaks become noticeable when the engine is shut down.
To remedy this situation, replace all the Norgren pressure protection valves with those of an improved design. The new pressure protection valves have an "A" stamped on the upper front surface, above the part number. See Fig. 1 .

Parts Required
Parts are available from the PDCs. See Table 1 .

Procedure
1.    Park the vehicle on a level surface, shut down the engine, apply the parking brakes, and chock the tires.
2.    Disconnect the batteries.
3.    Drain the air system.
IMPORTANT: Clean the AMU and the area around it of all dirt and road debris before removing any modules. Failure do to so can result in dirt or road debris getting between the modules and their seals, causing air leakage.
4.    Loosen the bolts on the L-bracket, which secures the air management unit (AMU) to the frame rail mounting bracket. See Fig. 2 . This will allow the AMU to separate, allowing for easy removal of the pressure protection valve.

IMPORTANT: Be sure to mark the air lines before disconnecting them from the pressure protection valves.
5.    Mark and disconnect the air lines from the pressure protection valves.
6.    Pull out the locking keys from both sides of each pressure protection valve and pull the AMU apart. Remove the pressure protection valves from the AMU.
7.    Place the new pressure protection valves in position and slide the AMU together.
8.    Push in the locking keys.
9.    Install all air lines as previously marked.
10.   Check that all the locking keys are completely pushed in.
11.   Connect the batteries.
12.   Start the engine, build the air pressure to the operating level, then check the AMU for any air leaks. Shut down the engine.
If a leak is detected, drain the air system, then check all the locking keys and the gaskets.
13.   Firmly tighten the bolts on the L-bracket, to secure the AMU in the mounting bracket. See Fig. 2 .
14.  Remove the chocks from the tires.