Emergency air circuit operation on an M800 series ( air / hydraulic )

[someoldmoose]

I promise I have done due diligence to answer this on my own. Perhaps I missed it while searching. If so, please direct to appropriate thread. If not, then I'm glad I asked.

How is the Emergency side of the air system charged on my M820 ? Is it a valve somewhere activated by the parking brake switch ? I guess the same question can be asked about the Service circuit also, how is it activated ? Just curious as to the activation itself regarding towing / being towed since if I am towing something with air brakes the Emergency circuit has to be pressurized to release the towed vehicles parking brakes. So therefore I wonder how the Service side gets pressurized when I step on the brake pedal. Just trying to learn a thing or two is all. Thanks for any info.

 

[simp5782]

I promise I have done due diligence to answer this on my own. Perhaps I missed it while searching. If so, please direct to appropriate thread. If not, then I'm glad I asked.

How is the Emergency side of the air system charged on my M820 ? Is it a valve somewhere activated by the parking brake switch ? I guess the same question can be asked about the Service circuit also, how is it activated ? Just curious as to the activation itself regarding towing / being towed since if I am towing something with air brakes the Emergency circuit has to be pressurized to release the towed vehicles parking brakes. So therefore I wonder how the Service side gets pressurized when I step on the brake pedal. Just trying to learn a thing or two is all. Thanks for any info.

Emergency side is air from the tanks at all times it has pressure. There is only the shut off valves for feed at the gladhand connections.

 

[sandcobra164]

Like Simp posted, emergency side on an M809 series truck will have air pressure to the emergency side gladhand anytime the truck has air pressure and the valve just before the gladhand is in the open position. Air pressure sent to the service side is controlled by the air pack which receives it's input signal from the master cylinder. You press the brake pedal and the air pack determines how much harder to apply the brakes. One thing I'm learned about driving an air over hydraulic brake truck (Deuce), If you pull a full air brake trailer, it will tend to apply it's brakes with very little effort being done by the prime mover. I towed an MK-14 trailer 30 miles one time when it was about 40F outside and my brake drums were cold to the touch. I don't think the shoes ever touched the drums during that trip. I tossed that out because depending on the load, your trailer may not be good for stopping your heavy M820. If the trailer is light enough, I'd release the emergency side and let my truck do the braking.

 

[simp5782]

I believe there is a bypass override on the air pack when air comes from the service side that allows it to activate as if the brake pedal was pushed. I know they are slower to grab when the truck being towed has air over hydraulics. not like air brakes.

 

[someoldmoose]

Thanks guys ! I thought that might be how the circuits worked but I couldn't find a solid answer even looking at the schematics in the TMs. Didn't see any valves for the emergency side and had a suspicion the air pack was the service side control. Thanks for helping an old dog learn a new trick. Gets harder to do every day.

Thanks for the diagrams too. I didn't see anything like that before ( TMs, etc. ) After my escapades last weekend I want to know everything I can about the brake system air and hydraulic sides. This helped a lot.