Hi, I have a M931A2. I was driving down the road everything was fine, then it started slowing down. Eventually I figured out that the spring brakes had applied. After reading above, the spring brake tank has no pressure. I pressurized through the drain line with a compressor, and the spring brakes release if I push the emergency spring brake override button on the dash. Simp5782 said "If spring brake tank is empty then the kn23000 check valve or the 3way valve there is bad." Can you tell me where these are located so I can find them and check them out? I see there is a PPV valve on the front of the wet tank and a ST check valve at the back of the wet tank. Are these the ones, or something different?
Right behind the drivers step. to the rear of the air cleaner. You will see it there. Little Valve. Will have a T fitting on it and one vertical looking half cylinder and one horizontal cylinder. The little line coming in from the top supplies air a secondary to that. The Rear large 1/2" line is green supplies air from the primary tank. The large red line going vertical under the cab supplies air to the linear valve on the parking brake handle. The forward line allows air to go to the spring brake tank
Remove the large 1/2" line that goes up under the cab from the T block. If there is no air from this then you need the front valve which is Check valve # 7 is part number KN25060. This is a 2 way check valve that picks air from either supply that is available. Sometimes it becomes stuck and blocks off the air completely.
If you have air on that line that you removed going to the parking brake then put the line back on the T then you need to remove the front line to the spring brake tank. if you have no air coming from it then you need KN23000.
If you replace KN25060 I would also replace Kn23000 while i was in there and keep the old one for a spare. It is the same one way check valve as the main and secondary tanks in case they fail or rust up.
Will post pics in a minute.
I will add this as well for a useful helpful tip in a bind. IF your spring brake tank does not have any AIR in it then you CAN NOT use the spring brake override. IF this is the case and you need to get it off the road you CAN remove the air line going to your shutterstat on your engine. You can then in turn remove the line going to the firewall that is the supply air line for the spring brake override to the dash board. The JIC fitting the shutterstat supply hose will screw onto the firewall port for the spring brake override switch supply on the firewall. This will supply air from the main tank to the valve so you can override the spring brakes and drive on to safety. Just keep an eye on your engine temps. Or if you have a #6 JIC to #6 union you can hook the line together and do away with the switch all together.
https://youtu.be/tHT5nA14AxI