Hey, I'm new around here just wanted to ask a question for those with a good knowledge of the Air Brake systems in their trucks... I apologize if I've placed this question in the wrong location.
This is a pair of diagrams from the locally distributed Air Brake Handbook (Government):
The question I have is whether the Tractor Protection Valve pictured is inverted... It looks like a TP-3 style Protection Valve to me; which would mean that based on the above diagrams, they are running the Service side through the Emergency ports.
Here's a cut-away of a TP-3 style TPV:
The way this image shows the unit, the following are the ports:
1 - Trailer Emergency
2 - Tractor Emergency
3 - Trailer Service
4 - Tractor Service
My understanding of the basic function - Emergency line (2 > 1 above) receives air pressure while the Trailer valve is open (Supplying air) on the dashboard, it pushes down the internal plunger which allows service air to flow from 4 > 3 when the service brake is pressed or the spike is activated. If the trailer breaks away or the trailer valve is closed (Not supplying air), the air pressure between the dash valve and ports 1 & 2 above falls too low and the plunger resets back to the above position, blocking the flow of service air so you don't dump your tanks.
I'm just looking to find out whether I'm reading this diagram wrong - if I've completely lost my marbles here by all means please set me straight!
Tractor Protection Valve
Discussion in 'Trucks [ Eighteen Wheelers ]' started by k2x5, May 25, 2016.
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Your red button "trailer supply" protects the system from a pressure loss. The trailer breaking away or losing your red glad hand connection does NOT cause the red valve to pop. It may take some while for the loss of air pressure to be enough for the red valve to pop.
Once it does pop you are correct as described, this causes no air pressure in the red emergency/supply side. The valve at the back of the cab then shuts off the blue "service" side. Think of the rear valve as a slave to the red button.x1Heavy, Oldironfan and AModelCat Thank this. -
Yep. Red button (trailer supply) is what protects your tractor air supply in the event of a trailer supply line break. The only purpose of the tractor protection valve you pictured is to stop air from flowing out your tractor service line when you step on the brakes bobtail. Its basically a pilot operated shutoff valve actuated by trailer supply line pressure.
x1Heavy, Oldironfan and scottied67 Thank this. -
The OP appears to be correct in his description of the tractor protection valve. In the diagram the spring on the valve should be towards the service side of the system. A small detail in the diagram but the OP seems correct.
The tractor protection valve is so named because it does indeed protect air pressure. As we all know it can be some time after the service line is opened until the trailer supply button pops. In a situation where the trailer emergency line is broke, air pressure must be conserved in order to operate a vehicle safely off the road. Once air pressure is dropped, by a line break for example, on the emergency side of the trailer protection valve the service side of the trailer is cut off. This conserves air pressure to allow a tractor to be operated until it is safely stopped off the road should an emergency line break or trailer become disconnected.
There is a short time after the trailer air supply pops and the tractor parking brake pops. If no attempt is made to conserve air pressure, a drive may not have the time to safely move a disabled tractor off the road. Hence the tractor protection valve.Last edited: May 26, 2016
x1Heavy, Oldironfan and Cetane+ Thank this. -
If i have a tractor converted to a dump truck can i eliminate this valve?
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You don't have to do anything. It only functions when the red button is pushed in anyway.
snowman_w900 Thanks this. -
Im having issues with the rear brakes locking up it has a bp-1 relay proportioning vqlv3.
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Oldironfan Thanks this.
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You may have bad brake Chambers and they may have the spring in them broken or the diaphragm is bad and the spring is just sprung.
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