m931a2 rear brakes won't release

Hello all my 931a2 just started this problem went to release parking brake dropped lever and brakes would not release also heard constant air dumping into air intake stack any one ever seen this problem, also thought it might be some kind of valve being stuck what are the valves inside the box under the drivers door? thank you

PMF5150 said:

Hello all my 931a2 just started this problem went to release parking brake dropped lever and brakes would not release also heard constant air dumping into air intake stack any one ever seen this problem, also thought it might be some kind of valve being stuck what are the valves inside the box under the drivers door? thank you

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Has it been sitting? If so. The brake shoes are prob stuck.

Being that you hear the air dump means your linear valve on your emergency brake handle is still applied and not releasing. The treadle valve bypasses air to the stack when the pedal is applied and the linear valve is engaged since the linear valve goes thru the inversion valve. Try tapping the linear valve with a rubber hammer while you jiggle the roller wheel arm up and down if that works. Replace it. Member swampdonkey has nos replacements

You also need to check that you have air to your spring brake tank. So pull that line off the drivers step tank and see if you have air coming thru it. If not then the kn23000 check valve or the 3way valve there is bad. You have to have air to the linear valve for it to be sent to release the spring breaks.
A quick way to check if your spring tank has air is push the spring brake release on the dash. No noise then the spring brake tank is empty. Don't trust the drain line to verify air pressure in that tank as they stop up.
I have had the 3way valves go bad alot just cause your gauges show air doesn't mean that tank has air in it.
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Hi thanks for your help let me tell you where i am at the truck has not been sitting for long i used it a couple of weeks ago plowing snow for 8hours no issues rand great ran it last night stopped for coffee and when i released brakes to leave it was dragging bad got it back to garage all rear wheels were hot steaming let it cool over night tried it again this am same thing no release of rears i tried the brake release on the dash had no effect brakes still locked up also tried what you said and tapped the lineal valve while moving switch lever nothing changed still pouring air into air intake i replaced the treadle valve about 3months ago for a diff problem but the truck was fine after that. if lineal valve is bad where can i get one oshkosh wants 300 for it ouch just a thought if lineal is bad could it be changed over to a dash brake release valve like on newer trucks? thanks

PMF5150 said:

Hi thanks for your help let me tell you where i am at the truck has not been sitting for long i used it a couple of weeks ago plowing snow for 8hours no issues rand great ran it last night stopped for coffee and when i released brakes to leave it was dragging bad got it back to garage all rear wheels were hot steaming let it cool over night tried it again this am same thing no release of rears i tried the brake release on the dash had no effect brakes still locked up also tried what you said and tapped the lineal valve while moving switch lever nothing changed still pouring air into air intake i replaced the treadle valve about 3months ago for a diff problem but the truck was fine after that. if lineal valve is bad where can i get one oshkosh wants 300 for it ouch just a thought if lineal is bad could it be changed over to a dash brake release valve like on newer trucks? thanks

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Make sure the tank below you drivers step has air. Im sure it doesn't if you didn't hear any air when you hit the override valve.

That allows air to go to the rear spring cans with the linear valve off.

https://youtu.be/ELcWO5Q2A1w

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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?

bevanet said:

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?

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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

Thank you so much! It was the KN25060. I'll get one ordered and the KN23000 too. The KN25060 had a plastic/rubber diaphragm inside that was hard and a circle cracked open. Somehow that prevented air going to the spring air tank. I took the diaphragm out and put it back on so I can at least move it around the yard. I know it will be safer when I get it replaced. Now I have to figure out what is going on with the CTIS. All the lights blink. When I press Hiway, it blinks for by itself for a bit but then they all start blinking again.

Thank you again!

bevanet said:

Thank you so much! It was the KN25060. I'll get one ordered and the KN23000 too. The KN25060 had a plastic/rubber diaphragm inside that was hard and a circle cracked open. Somehow that prevented air going to the spring air tank. I took the diaphragm out and put it back on so I can at least move it around the yard. I know it will be safer when I get it replaced. Now I have to figure out what is going on with the CTIS. All the lights blink. When I press Hiway, it blinks for by itself for a bit but then they all start blinking again.

Thank you again!

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Bad control unit more than likely. It can be rebuilt.

Subscribed; good to know stuff in a pinch.
Thanks Wes.
JC

Can you tell me where the CTIS control unit is located or a picture? Do I need a kit to rebuild it, or is it a matter of cleaning and lubing? The truck only has 2800 miles so it should not be worn out, but maybe froze up or dried out.

bevanet said:

Can you tell me where the CTIS control unit is located or a picture? Do I need a kit to rebuild it, or is it a matter of cleaning and lubing? The truck only has 2800 miles so it should not be worn out, but maybe froze up or dried out.

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It is on the side of your shifter where it plugs into. It has diodes. check with Andy6x6 he did some rebuilds on it. Should be a thread somewhere.