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trouble finding air leak
I have been trying to chase down a resent air leak on my H without much luck. I will air up in the morning and hold all day without any noticeable drop. No issues while driving. Over night, primary, secondary, and act will all drop equally about 25 lbs. over the next few days the air will continue to drop equally at about the same rate. The Acc will drop to 0, but the primary and secondary will stop dropping and hold at about 70 lbs for weeks. The only other thing I noticed, which seems odd, is that when I press on the brake all three air reading will have the standard drop equally. When I do a break check it performs as it should, but thought the acc acting with the primary and secondary was odd because i thought it was separated from the other two. I do not know it I am chasing one problem or two different problems. I have been staring at the air schematic for weeks and these indications do not make any sense to me. Tried the soapy water check as well as ultrasonic on all the fittings I can reach. (Do not have ramps available to get fully under bus). Any ideas on where to focus on
Thanks
Alex -
25 Replies
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It sounds as it a protection valve or check valve is not working as it should.
The accessory system can go to zero while the primary and secondary retain pressure, but when all gauges move at the same time when the coach is fully charged something is wrong.
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Jon,
i may be chasing three issues. My thought was for the primary and secondary to be leaking equally, it would be something common on the demand side of those tanks, and for the acc to go down with them, the acc check valve or protection valve for that tank may be bad and also have an additional leak on the demand side of the acc which pulls that down to 0.
thanks
Alex -
A leak that brings down both the primary and secondary simultaneously can relate to the parking brake. Just remember when the bus is parked with the brake on there is no air pressure past the relay valve to the parking brake chambers so it can be in the other parts of the system up to the relay valve.
Go to the Prevost Car web site, maintenance and parts, and publications. From there you can get your shop manuals and pneumatic diagrams to help with tracing air flow.
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what is confusing is P&S dropping to 70 lbs and holds at that point.
Thanks -
IMO, Sounds like there are 2 problems. #1) A leaky something? on the aux system(which in my world is normal), and #2) a bad aux system pressure protection valve.
There are two pressure pretection valves(PR-2) on my bus. One on the manifold block that insures at all times a minimum pressure, roughly 75 psi, in the suspension air system in the event that a pressure drop occurs in either the suspension air system or aux air system. The other valve is installed on the aux air tank, and insures a minimum pressure, roughly 75 psi, in the aux system in the event that a pressure drop occurs in either the suspension system or braking system.
Here’s a pdf manual for PR-2, PR-3, and PR-4. http://www.bendixvrc.com/itemDisplay.asp?documentID=5986
PR-2 valves range in price at FindItParts.com from $13 to $40. They’re adjustable too.
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thanks Don, I will have to get to the acc tank to check the valve
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Alex,
As you can tell, our coaches were on the assembly line the same time. Here’s what I’ve discovered in my first 18-months on the road with my Marathon H, mindful that Jon & Tom have forgotten more about Prevosts than I know!
My check valves on my F/R also stop around 60 psi when something on my Aux side is leaking. However, the F/R is tied into the Aux tank up and until it reaches that 60 psi, when the F/R check valves kick in and separate any further leaking out of the brake system.
If your coach is set up like mine, a few of the biggest leaks I’ve find, rectified and/or fixed, were my air-bed, mini-airbags/check-valve, and the pneumatic passenger swing-up step.
Without going into great detaiil, I located the airleak underneath one of my closets that was coming from an SMC vavle. Rather than spend several hundred dollars on a new SMC valve, knowing that there are numerous places an air-bed can leak, and based on the sound advice of another fellow PC member (Big Al) I simply plugged my airline to the airbed and I now open my bed using gas struts, which works great. However, the air-bed works from my Marathon compressor, not the Prevost side of the house.
However, the two biggest leaks on the Prevost side were my passenger side air step and my check-valve/mini-airbags in the engine compartment.
I the rebuilt the cylinder for the passenger step which my wife uses all the time (Not the air-slide, but the pneumatic swing out step). This was a $20 fix in O-Rings and it’s been working great ever since.
I also replaced the check-valve for my two mini-airbags in the engine compartment, as well as the two mini-airbags.
Now, my Aux will stay at 105psi, along with my F/R tanks, for weeks.
I know that numerous other items can leak, like the seat, or push-on fittings below the driver’s seat, but so far I’ve not had huge issues with either one of those, although I did replace a push-on fitting with a compression fitting under the driver’s seat last winter.
Again, I’m still very new at this so someone with much more experience can chime in and correct me if I misstated something.
Hope you can locate the leak …
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I believe the issue goes beyond just the protection valves.
Leak free brake systems should maintain their pressure even if the accessory system drops to zero so the problem is likely related to multiple components. My coach can have both brake systems remain at full pressure for a couple of months after the accessory system has lost all its pressure.
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Jon,
So your F/R brakes remain at 120 psi, despite the Aux pressure?
I was told at Prevost that those three air systems were tied into each other, up to a certain psi where the F/R brakes disengage from the Aux tank. So are you saying this is not the case?
On my bus, when I had the Prevost Aux leaks, my Aux would go done to zero within 48 hours and my F/R brakes would stay at around 60-70psi.
Now, my Aux system stays between 105-115 psi and so do my F/R brakes.
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Tim,
Our aux. tank is behind the front bumper that controls all the air filled items (slide seals, dump valves, genny bags, 2 air doors, etc.). When the coach is stored indoors, which it is now, I turn off the aux. compressor as it will probably run once every six hours for 1.5 min. to keep everything ‘puffed up’. :o) I will turn it on once or twice a week to keep everything ‘exercised’.
Our F/R air pressure stays at 120-125 lbs. even if the aux. compressor falls to zero.
We don’t have storage under the bed per se, we do have a shower pump that needs cleaning periodically and I’m sure there are electronics under there. We also have a manual pop-out step for the passenger that slides out easily with a ‘one-hand-pull’.
Our sewer hose is also manual (no air).
Safe travels,
Mark
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I think we’re talking about two different things. My Marathon Aux compressor has nothing to do with the Aux gauge on my Prevost Dash. My air bed, bathroom pocket door, and gen airbags are controlled by my Marathon Aux compressor (no slides).
Everything else is aired up by the Prevost compressor, which controls the F/R brakes, as well as the Aux accessories on the bus.
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Common areas for leaks in the brake system(s) are: 1) the R12 & R14 relay valves; 2) the emergency/parking brake valve(RD-3). These componets have o-rings in them that are prone to leak w/age.
https://prevostcommunity.com/forum/threaddetails.asp?Category=Pneumatic&ThreadID=6143
Based on what I’ve read here on PC the last 3+ yrs, my bus is like most, its aux system has sub-systems that leak. My bus loses 8 psi/hr. Not too shaby. When I bought it that # was about 25 psi/hr. But it doesn’t take down the pri or secondary brake tanks. Like others have stated, my brake tanks leak about 1 psi/day.
The aux tank has two air lines, 1 line, the supply line, comes the secondary brake tank(front) thru the secondary brake tank mounted one way check valve and at the other end thru the aux tank mounted pressure protection valve. The other line , the delivery line, delivers air to all the sub-systems thru the manifold I mentioned in my previous thread here.
I just can’t figure out how all three systems could be taken down together.????????????
If your talking about a fourth air system that only Marathons have? I’m clueless!!!
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Tim,
My Prevost aux goes to zero over time, but has no affect on the primary and secondary brake air tanks. Some will get a little confused here because of the differences in our Marathons.
I have one aux compressor that provides air for the Marathon side only, yellow knob closed or to both, yellow knob open, for Prevost aux. Some Marathons have two compressors, mine did from the factory, a small 24v along with the 120v. I removed the 24v when it went TU, don’t need it.
I don’t mix the two systems and leave the compressor on at all times when using the bus. The reason being, the aux compressor makes lots of water and I don’t want to induce that moisture into the Prevost aux.
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Tim,
What Tom said because his is a Marathon like yours.
Regardless of conversion however the chassis accessory air could go to zero and the primary and secondary brake systems remain at whatever pressure they were when the engine was turned off. If they do not, but track the accessory system down there are issues with protection or check valves.
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Got it!
Tom, I don’t mix my Marathon Aux compressor with the Prevost side air either and I only have the one 12v for Marathon.
So Jon, right now since I stay at 105psi for weeks on the Prevost side, I’m not really sure how to track down if it’s a check valve in the F or R right now, or whether it’s that important since all my DOT brake checks don’t lose any psi over the course of several minutes.
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