Air brake assist questions

The M-817 I bought has air lines front right and left as well as rear right and left. Can someone help me understand how they are intended to work. I do not have any air controls in the cab. All my other trucks non military had dash valves as well as the column mounted trailer brake control. Not having any controls leaves me with these questions. Thanks....

https://www.steelsoldiers.com/showthread.php?121483-TM-s-for-M809-Series-5-Tons

Read the one that ends with -10.

PeteNora said:

The M-817 I bought has air lines front right and left as well as rear right and left. Can someone help me understand how they are intended to work. I do not have any air controls in the cab. All my other trucks non military had dash valves as well as the column mounted trailer brake control. Not having any controls leaves me with these questions. Thanks....

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The 817 has air over hydraulic brakes. Not straight air brakes like common road trucks
The passengers rear and drivers front are emergency gladhand for towing a trailer or for air supply to the truck for towing.. They should have manual valves on them

The passengers front and drivers rear are supply ports to actuate the truck brakes if it's on the receiving end and to actuate any trailer/truck that needs air to activate its brakes.

The military did not do push/pull type tractor valves on the 5 ton trucks except the tractors for their 5th wheel air. The rear glandhands still have just valves. The larger trucks and FMTV trucks all went to standard air brake system parts for parking/tractor valves

simp5782 said:

The 817 has air over hydraulic brakes. Not straight air brakes like common road trucks
The passengers rear and drivers front are emergency gladhand for towing a trailer or for air supply to the truck for towing.. They should have manual valves on them

The passengers front and drivers rear are supply ports to actuate the truck brakes if it's on the receiving end and to actuate any trailer/truck that needs air to activate its brakes.

The military did not do push/pull type tractor valves on the 5 ton trucks except the tractors for their 5th wheel air. The rear glandhands still have just valves. The larger trucks and FMTV trucks all went to standard air brake system parts for parking/tractor valves

Click to expand...

Thank you for the information. I have tried to read the manuals and I find them hard to follow. I pretty much understand the trucks air over hydraulic operation. I don't have this home yet so my questions are based on the quick trips to the truck doing basic items to get it home once the snow goes.
I am always alone so its hard to operate and verify my concerns when there is not another set of eyes and ears. The one concern is that the truck builds and holds air pressure ok. When I apply the foot brake after a second or so of application I hear an air leak. It depletes the air fairly quick so the buzzer comes on. I am assuming at this point when hydraulic pressure is applied to the air slave unit air is assisting the hydraulic part and possibly leaking when doing so. Or does the air also go to the front and gear glad hands at the same time? Next time I go to tinker on it I hope to have a friend go with me or I will have to take a pedal compressor to apply the master so I can look myself.

The FRONT glad hands are used when BEING TOWED.
The REAR glad hands are used when TOWING.

dmetalmiki said:

The FRONT glad hands are used when BEING TOWED.
The REAR glad hands are used when TOWING.

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Understood. So if I am correct when the truck builds up air pressure one line should be charged. The other must charge when the brake is applied. I am trying to understand what applies the pressure or determines the pressure when braking? Without a cab control I am trying to figure how this system works.

PeteNora said:

Understood. So if I am correct when the truck builds up air pressure one line should be charged. The other must charge when the brake is applied. I am trying to understand what applies the pressure or determines the pressure when braking? Without a cab control I am trying to figure how this system works.

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Right. When the truck tank has pressure the “emergency” glad hand in the rear will always have pressure. This is where you can connect a hose for airing up tires etc. The “service” glad hand in the rear will have varying air pressure when the truck brakes are applied controlled by how hard the pedal is pressed (providing everything is working properly). It could be that the line going to the rear is broken or the service valve is open generating the leak you referred to.


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The really basic way I can describe the system when the truck is in operable status. The air compressor builds air which is stored in the air tanks. The brake pedal actuates a hydraulic master cylinder which sends hydraulic pressure to the "Air Pack". It boosts hydraulic brake fluid pressure by allowing air pressure to feed into the back half of the air actuated cylinder in a metered fashion. As a fail safe, the master cylinder can supply hydraulic pressure without assist although it will be at a much lower pressure and stopping distances will increase. You only need the service line hooked up to an M809 series truck. The air will transmit to the back of the "Air Pack" booster and apply the brakes.

simp5782 said:

The 817 has air over hydraulic brakes. Not straight air brakes like common road trucks
The passengers rear and drivers front are emergency gladhand for towing a trailer or for air supply to the truck for towing.. They should have manual valves on them

The passengers front and drivers rear are supply ports to actuate the truck brakes if it's on the receiving end and to actuate any trailer/truck that needs air to activate its brakes.

The military did not do push/pull type tractor valves on the 5 ton trucks except the tractors for their 5th wheel air. The rear glandhands still have just valves. The larger trucks and FMTV trucks all went to standard air brake system parts for parking/tractor valves

Click to expand...

Thanks as always for having good thoughtful easy to understand answers and not just throwing out RTFM!

I know sometimes folks read the manual and still have questions that's why we ask those more knowledgeable.

sandcobra164 said:

You only need the service line hooked up to an M809 series truck. The air will transmit to the back of the "Air Pack" booster and apply the brakes.

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Good point and if it was ever to be lift towed it would be the same just the service line on the 809 series. If one was to connect the emergency line and air up the system plus lift tow then the sprag would come into play and not be good.