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Pressure protection valve
On the Prevost AUX air side we all have a Pressure Protection Valve.
Its purpose, keep appox 70lbs of air on the Prevost AUX side to keep a reserve ready when you turn your ign key to on. This is enough air to quickly make ready your engine belts tight.
Lets pretend you have some Prevost AUX air leaks.
Pressure protection valve should maintain this 70lb reserve held in AUX tank if the valve seals when it hits 70 psi.
In my case sometimes I would keep the reserve appox. 2-3 days. Other times leaks to zero psi….. valve needs to be replaced.
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11 Replies
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I disagree BA.. The protection protection valve prevents air flow into the auxiliary air system until the braking system has been supplied enough air to function, usually around 70 psi. Doesn’t have a thing to do with maintaining aux air for the belts.
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Truk post...Doesn’t have a thing to do with maintaining aux air for the belts.
Agree, One of the functions of this valve is to hold a supply of air to belts if in fact needed at engine start. That air to belts “helped” so as to not throw the belts off passenger buses was Prevost thinking.
We don’t have belt problems with the seal tight mini bags,hoses and a brand new fancy check valve we each installed last year.
As explained to me the valve will close to maintain supply in Prevost AUX tank at appox 70 psi. and should hold.
In my case sometimes it did and more often did not….. which was confusing.
I still have a small leak on Prevost AUX side going 2 days to leak down. I’ll be leak free someday.
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x2 what Tom said.
Here’s a picture of what a PR-2 protection valve looks like. My bus has 2, one as pictured in the steer compartment and another on the aux tank itself.
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Very very important for all to understand protection valves are to protect the braking systems. They should never allow air to flow to an aux system until the brake systems have at least 70 PSI.
Even if you never get beneath the coach to drain the air tanks in sequence to verify the check valves and protection valves are working at the very least upon start up with low or no pressure in any of the systems you should not get air pressure into any component or system until the primary and secondary systems have at least 70 PSI.
If you do not have an aux system air pressure gauge look for other clues to tell you air is not building in the aux system until the brakes have pressure. I have an aux pressure gauge, but even if I did not I would know when aux air is building because my seat will not get air pressure until the brakes have pressure.
With properly functioning protection valve(s) you can experience a catastrophic loss of aux air pressure such as a failed air bag and still retain fully functional brakes.
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Good post as a reminder …
Had mine replaced when I zeroed out a couple years ago …
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Thanks guy’s for adding more detail about functions of important parts of our buses and understand why they are installed.
This should draw attention to owners of buses where leak down of air systems can be traced to a valve, easy to replace, and not very expensive, under $75 bucks.
Tim, why was your Pressure protection valve replaced ?
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I cannot answer the question regarding Tim’s replacement, but I suspect apart from age the biggest cause of failed protection valves is moisture in the pneumatic system.
I know we sound like broken records when we discuss certain issues on the pneumatic system but moisture within the system really messes up the valves within the system. In the case of protection valves I suspect corrosion damages the spring within the valve and compromises its ability to function as required.
The importance of the protection valves is they allow you to have control of your coach in the event of a pressure loss in part of the system. By maintaining pressure to keep the braking system operational you have flexibility in determining when and where to stop for safety or for repairs. If the protection valve fails and air gets exhausted from the pneumatic systems your parking brakes are going to apply and unless or until you can re-build pressure your parking brakes will remain applied. Imagine that happening in a center lane on I-75 in Atlanta at rush hour.
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So why are Pressure Protection valves not added to the length of service list of items that should be replaced when air bags, norgrens, been changed out at a suggested 10 year service life ?
Seems to me in Missouri (BTW, the show me state) that “we” have missed a important safety valve to assure one or maybe two braking functions are available in case of an emergency.
In my case my clue was intermittent holding at 70 psi. I could of had the problem in past and did but never paid attention. The more you work on these monsters ……the more you learn.
AL
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There are certain things you can replace as needed. A protection valve is kind of like the suspenders to go with your belt. First, it is easy to diagnose when it isn’t working if you do a pre-trip brake check, and second it really only is needed if something else in the pneumatic system leaks catastrophically.
Relay valves can fall into that category also. If the brake system(s) go to zero pressure in a short period of time such as overnight, and you are confident other potential leaks such as the pressure gauge fittings or the brake treadle are not leaking they could be an “as needed” replacement.
We all would go broke replacing everything that had the potential to fail without some evidence to suggest a life span. We have ample anecdotal evidence to suggest things like air bags, brake chambers and Norgren valves have a life span, but due to their reliability few of us have ever had a protection valve, relay valve, check valve or foot treadle failure. Unless we have a pneumatic system dripping wet at which point everything should be suspect.
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At the time, January 2016, I was chasing down a persistant air leak that took my F/R brake air pressures to zero within an hour. Prevost DL stated that the pressure protection valves could be leaking and replaced them.
Turned out the air leak was behind the dash on the air line going to one of my air gauges … which Prevost DL found.
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Al,
Makes sense to Me. The name of the valve even suggests that it should be checked out. “ PROTECTION “ Good post Al. I think that when doing a pneumatic re-build they should be changed out on a older coach anyway. Along with the Aux system protection and moisture separation units.
Jon,
How many do We have ? So far I know there are (2) P12’s and (2) P14’s and (1) PR2 in drivers bay and another PR2 on AUX that Don mentioned. How many more are there that I do not know about ?
Thanks in advance
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