air tank pressure protection valves

[David Little]

 My 1995 Beaver Patriot on a Spartan chassis has a pressure protection valve between the primary and secondary air tanks. The wet tank is the top tank and is fed by the engine compressor. Once that tank reaches a certain pressure, the pressure protection valve opens, and air feeds into the secondary bottom air tank. If pressure get too low in the secondary tank, ( a blown air bag, etc.) the valve will shut and trap the air in the top tank, (Primary, rear brakes). My valve is leaking badly out of the vent holes on the bottom and around the joint of the two halves. The diaphragm is shot. I have to replace it. Problem is, I do not know what the original pressure to OPEN the valve is, and what the SHUT pressure is. the numbers on the valve are N15759DD.  I can find many valves with that number, but they are set to open at 95 PSI and close at 90 PSI. I see other ratings for open at 65 and close at 45. I bench tested the valve and it seem to start to open at about 60 PSI. It is leaking so bad that I do not trust the bench test for open or close pressures. I called Spartan and they are useless on this matter. Does anyone have any idea at what pressure the OEM valve was set to open, and close at?  It is a simple two tank system, full air brakes, (drum brakes) all around with 4 air bags.

[Eric Maclean Co-Admin]

David
I'm not sure about your tank configuration but if you only have two tanks then the first tank is most certainty a dual tank meaning it looks like a signal tank
But it has a divider baffle in the center with a check valve in it making the first half your wet tank and the second half the primary tank .
After 1972  air brake systems by federal law were required to have two separate air feeds to a dual air brake treadle valve so you effectively have two separate brake systems ( front and rear ).  For safety reasons if one system lost all its pressure you would still have the second to bring the vehicle to a stop.
With the two tanks the second one is the secondary tank not an accessory tank the valve you are seeing is the primary tank brake protection valve which accomplishes the protection feature between the primary and secondary tanks. So in the event of catastrophic pressure lost on either tank the valve would stop the remaining air in the non effected tank from transferring and escaping leaving you with not brake air pressure.
This is why those air pressures seam high to you that valve is set higher to maintain enough air pressure to effectively operate the remaining or no effected brake system.
That valve is speed to open at 95psi and close at 75psi

The air pressures you have on that valve are correct for that application, again this is a brake protection valve not an accessory protection valve.

With that said you should also find an accessory protection valve on the secondary tank leading off to run the accessories such as air bags ,air doors ,HVAC etc this protection valve will have the lower air pressures your mentioned as it's job is to protect and maintain the secondary tank air pressure above a minimum air pressure for proper air brake operation usually closing at  70 to 75 psi which is enough to operate the second set of brakes usually the front
The two different cut off pressures are what differentiate the primary and secondary . Usually the primary system operates the larger drive axle brakes and the secondary provides air for the secondary ( front) brakes and accessories.

https://www.fleet-products.ca/kn31010-g-95-75-psi-inline-pressure-protection-valve-n15759dd-90555396/?srsltid=AYCX9vJuWrF8EF2Dyqn8-5lk59Y3FosQ3Yr4BlotiecYIfZM8MV8LZ15


Hope this helps
Eric