Tractor Protection Valves

Ok, I think you are getting the tractor protection valve and the dash park brake valve mixed up. The dash park brake valve is not a tractor protection valve. The tractor protection valve does close instantly when there is a break away. This valve will prevent service air brake pressure from going out the valve. With the trailer park brake button still pushed in, air will continue to rush through your dash valve until the drop in pressure is between 65 and 35 psi. The reason the pressure "kick out" of the buttons is so low because during an actual breakaway the trailer emergency system would normally be charged to 120 psi or so then it would drop very rapidly when the line is opened. You really do not have too much pressure at an open line, just alot of flow, so that is why the button will pop out at such a low pressure. When driving and you lost your trailer the button would pop out well before your truck system got down to under 60 psi.
[ame]http://www.youtube.com/watch?v=-1vA1EnNJZ4[/ame]

 

usually trailers are with emergency valves and when you disconnect supply line (red gald hand), ore if you pull out trailer supply button trailer park brakes are actuated with trailer emergency valve.

valves installed on new trucks are to prevent dumping air from trucktor if supply line is broken.

 

Tractor protection valve is designed to shut off the air flow from the blue line (service) when the there is no air pressure in the red supply line (emergency), when your dash red supply button is up.

So when you press on the foot brake the blue line will have no air flow going out the blue line. It is not designed to stop air lost of emergency line when there is a break away. That will be the job of your dash valve.

 

This is exactly what I meant.


BTW, the 1 extra smaller port in a 5 port valve is usually a connection for your foot brake pressure guage in the dash and the stop light switch sensor.