So, your air dryer pressure relief valve keeps popping off randomly on your diesel RV or truck equipped with air brakes. Or your air pressure is not consistent. Mine ranged randomly from 95psi to as high as 125psi. There are several problems which can cause this. The a bad relief valve, the governor on the air compressor being bad (Common problem. They are cheap so carry a spare) or clogged governor control lines. But a hidden problem could be inside the Haldex air dryer. Especially if it is getting a little old. (Mine is 23 years old. Impressive it lasted that long. A shout out to Haldex!!)
I have the Haldex N50011h air dryer. Hidden inside the air dryer is a shuttle valve which provides control air to the governor. The shuttle o-ring or rubber washer degrade over time. My rubber washer was toast. Failure causes the governor to not cut out when it should and overpressure the system. For this air dryer, the part number is Haldex DQ6031, Turbo Kit for Pure Air Plus (see pic). There are similar shuttles inside most air dryers as the dryer usually provides control air to the governor. You can replace the air dryer for about $350 or purchase the shuttle kit for about $30 (I found mine on eBAY). It is a pain to get to, but will fix the problem. To see the shuttle valve, remove the air dryer spin-on filter and internal screen . Look down the hole to see a a bit of the shuttle. To get to it, you have to remove the air line adaptors opposite both sides of the shuttle hole. There is a spring, so watch it as you remove the adaptors. Remove the screw and rubber washer out one side and the shuttle out of the other. Clean the hole thoroughly. Inspect the hole for excess corrosion or scratches. If that is bad, sorry you need a new air dryer. Lightly lube the hole. The rubber washer is screwed to the opposite end of the shuttle and must be inserted opposite the shuttle. That screw is painful to put in as you can't see it where it meets the end of the shuttle. It should slide easily back and forth about an inch when assembled correctly. Reassemble in reverse. Put the money you save in the gas tank and hit the road again. Good Luck!!!!
