Replacement Of The Priority Valve On The M35A3

[glcaines]

I finally got around to replacing the priority valve on my M35A3. This procedure is not in the TMs, but it is straightforward. I was tired of banging on the priority valve with a hammer prior to driving to get the valve to release so that air pressure would build up. The purpose of the priority valve is to conserve pneumatic pressure to the brakes in the event of a failure in other less critical systems, such as pneumatic assisted steering, etc. The valve is located on the top of the air tank on the passenger side of the truck – impossible to miss. I purchased a new unit from Erik’s for $25.00, P/N KN-31000 (12450226). Erik calls it a ‘Pressure Protection Valve’ but the TM calls it the ‘Priority Valve’. You will notice from the photos that the replacement does not look the same as the original, but is a direct replacement for the original. Accessing the priority valve for removal is easy and straightforward. No need to jack the truck up. The only tools required are two crescent wrenches, hex wrenches, and some Teflon tape for re-installation. The whole operation only took 45 minutes.
Step one is to release all air pressure from the system and then remove all three connectors for the plastic tubing connected to the priority valve. Two plastic lines feed into a T on the end of the valve facing the rear of the truck. The other one connects at the front end. Make sure you don’t mix up the two that connect into the T. Once the tubing is disconnected, unscrew the entire priority valve assembly, including fittings from the air tank. I highly recommend taking a photo of the removed assembly because it is critical that the connectors are screwed into the valve at exactly the same angle as original. Note that the original valve has ‘IN’ and ‘OUT’ marked on the valve. The new valve has the openings labeled with arrows indicating air flow direction. It is critical that the new valve be oriented exactly as the original.
The next step is to plug the two holes on the top of the new priority valve with the plugs provided. Sealant is already present on the threads. The plugs are tightened with hex wrenches. I recommend removing the fittings from the old valve and installing them on the new valve one end at a time to make sure there is no mix-up of connectors. Use Teflon tape to seal the threads. Once the connectors are installed on both ends of the valve, the valve and connectors are ready to paint. I painted the entire assembly with Rapco 383. Once installed, I touched up scratches. The new valve has exposed zinc-plated screws that could rust, so I painted the unit.
The final step is installing the priority valve onto the air tank and this is accomplished with more Teflon tape. This is where you find out if you oriented the connectors into the priority valve at the correct angle. The new valve is configured slightly different than the original and the body of the valve will hit the air tank as the unit is screwed in if the angle is incorrect. When completely screwed in, the body of the new valve is almost touching the air tank.
After installation of the new priority valve, I was very happy. With the old priority valve installed, it took up to 5 minutes for the pneumatic system to build pressure to 120 psi and this was for situations where I didn’t need to beat on it with a hammer to get it to release. If the truck had been sitting for several days I almost always had to bang on it. With the new priority valve installed, it only took 27 seconds to build up to 120 psi from completely empty tanks. No more banging on the priority valve!

 

[FireFighterHill]

Great write up. Do you know which other military vehicles share this part?

 

[glcaines]

Great write up. Do you know which other military vehicles share this part?

I don't know what other MVs use this part, but I would suspect the trucks with air assisted steering probably would.

 

[gimpyrobb]

I was wondering if the A3s had these! On all the A2 trucks with the priority valve, its been mounted on the firewall. All the A# trucks I've seen, these were not apparent. Looks like they are near the air tanks huh? Good to know! Great thread.

 

[gimpyrobb]

One other thing, do you plan on taking the old one apart and inspecting it. Maybe you could see whats wrong and keep it as a spare once its fixed?

 

[glcaines]

One other thing, do you plan on taking the old one apart and inspecting it. Maybe you could see whats wrong and keep it as a spare once its fixed?

I do intend to dismantle it, but I haven't done so yet. I did look in both open ports and it appears to be spotlessly clean inside. I half expected to see a lot of crud or goo in it. Dismantling should be easy as I think there are only four screws holdig it together. I'll post the results.

 

[Deuce007]

Ever since I got my truck in July 2011 the priority valve has leaked and I am just curious as to what if any problems that might cause.

 

[glcaines]

Autopsy of the Old Priority Valve

Tonight, I decided to open up the old priority valve I removed from my A3. Removing the four corroded metric hex-head screws was straightforward. As you can see from the photos, the cause of the failure was evident. The valve body, plate, spring, etc. were all severely corroded due to water entering the valve. It was also evident that it had been corroded for a very long time. I was amazed that it worked at all, even with beating on it with a hammer. The inside was dry. After I picked up the truck in Sparta a few years ago, I replaced the air dryer as it was in very bad shape, although I'm certain it could be cleaned up. The cartridge was toast and there was corrosion and rust on the exterior, likely from salt on the roads in the winter. The heater was also burned out. I found an inexpensive NOS one on Ebay. I saved the old unit for possible rebuilding later. The air in the pneumatic system is now completely dry. When draining the tanks, there is never any hint of moisture coming out.

In my opinion, this priority valve is very poorly constructed. It should have been constructed entirely out of corrosion-proof materials. One interesting note is that there was no gasket under the cover. It could be that water got under the cover, as this internal portion of the valve was the only part corroded. Looking into the ports where the air lines were connected showed no signs of corrosion. Anyone dismantling one of these should be careful as the spring is compressed and parts will go flying if you aren't careful. As inexpensive as the replacements are, I would recommend anyone with an A3 replace the priority valve, and possibly other trucks as well. I would first make sure the air dryer is functional, however, because it is likely that the new replacements are constructed of the same materials as the old one.

 

[Deuce007]

The one on my truck leaks around the top where you said there was no gasket so about a year ago I put some Ultra black silicon sealer and that didn't help at all! Now I think that I will just order a new one in the hopes that it will be the solution to my low air buzzer alarm always buzzing when I turn the steering wheel.