[Archived]

Gordon, Your vintage coach is within the age range when the level low system logic was changed. Early Prevost coaches required the accessory air pressure to be at around 35 PSI or higher, and if it dropped below that level the system would exhaust tag axle air bag pressure, which would allow a drop in the rear.

Later vintage coaches would allow accessory air to drop, even down to zero, and air would remain locked in the air bags.

Step one is to determine what system logic your level low system has. Bring up coach pressure, turn off the key, somehow bleed accessory system air pressure and if the accessory air pressure drops to zero and the tag axle air bag pressures are not exhausted you have the later system logic. Knowing what system logic your coach has is essential to begin trouble shooting.

FWIW to address the question about why setting the level low in a certain position impacts the leveling two questions need to be answered. Your leveling system should be disabled with the key off. Verify this, or confirm the system still functions with the key off. The second question is did Marathon modify or supplement the level low system with an additional leveling system such as an HWH control?

[Ronald]

Hi John

Thanks for the response, I do have the later system,there is a leak on the Aux system in the rear that i need to rectify but its very difficult to access, however the Aux system can be at zero and the coach rear will hold up for a long time, in fact the driver side and front end will remain aired up for weeks.

My Level low system will not function with key off, i have replaced the up/down rocker switch previously to rectify a fault with the front of coach not lowering, that is now all good.

I do not have any other leveling system such as the HWH, only Prevost Level Low is installed.

i intend building a set of ramps to gain better access and then isolate each airbag on the passenger side and pressurize to attempt to isolate the leak , bags are less that 2 years old but i appreciate that they could still have an issue.

To assist in the short term i have installed a Aux air comp into the steer compartment ( Marathon forgot to fit mine ) that i can operate from inside along with an Aux air pressure gauge dash mounted, with this i can level the coach but its not the correct answer.

Apart from isolating each airbag i was wondering if i had missed any other obvious check ? also i am interested in understanding why when level low selector is set to drive the passenger rear will drop 1.5 inches overnight but when left in passenger select position the drop rate is greatly reduced.

Appreciate any advice you can offer with this

Thank You

[gordon]

The short version is with your coach and system logic the problem is likely limited to everything from the 5 port valve on the bulkhead forward of the drive axle, back to the tag air bag and everything in between.

Start with the low hanging fruit. Make sure none of the fittings within that range are leaking. Check to see if air pressure is escaping the Norgren valve spools sideways by looking for soap bubbles at the end caps.

Check the air bag fittings. Check the 3 port valve on the frame up between the tag and drive. That valve is always open except when the tag is lifted and then it shuts off air flow from the drive axle air bags and exhausts the tag air bag. Make sure air is not escaping from the short exhaust tube.

When I say check the Norgrens they can leak in two ways. One way is through the path they are supposed to block when the key is off. You can only check that by making sure air does not flow past them by putting air pressure to the inlet side and seeing if it goes down. If it does, pressurize the outlet side. If it does not go down the Norgren is the problem.

I built a hose with compression fittings on one end and a pressure gauge on the inlet side plus a ball valve to I could shut off air to or from the hose. It is laborious checking that way, fitting by fitting if nothing shows up with soapy water but that’s how I do it.

The hard part is if all points to the air bags they have to be pressurized on the bench because soapy water is not going to find leaks around the bottom, and a ultrasonic leak detector doesn’t work well on rubber.

The approach I take is if my level low system components are 10 years ol and show signs of the leans I do not bother wasting time chasing leaks because they are going to pop up everywhere sooner or later as the rubber ages in the valves or air bags, so I just replace everything and start again.

DO NOT GET BENEATH THE COACH UNLESS IT IS FULLY SUPPORTED AT THE CHASSIS SUPPORT POINTS. THIS IS ESPECIALLY TRUE IF YOU ARE MONKEYING WITH THE SUSPENSION

If your accessory air system has ever had moisture in it the probability the Norgren valves are leaking is very high because the highly polished raw aluminum valve bodies will corrode and leak in a very short time after exposure to moisture.

[Archived]

John, Thanks for the info and ideas, i will be under it at the end of the year when i can get it blocked up and start to rectify this issue,and will follow the complete system as you have described, I will start with isolating the tag and see what happens then with the drive, I like your idea and tool to check with the gauge and ball valve etc.

I have just serviced the coach and replaced the air dryer along with the R12 and R14 valves, and all flexible brake hoses i did not see any evidence of moisture when draining tanks etc but as i do not know when any of these items were last replaced i am starting to replace components that have the capability of leaving me stranded on the roadside.

this winter i will also replace all brake chambers as whilst servicing last week i was fitting cable ties to the actuator rods to measure / check brake travel, i found that the TAG brakes were backed right off and not actually doing anything, after adjusting i then found that they would not release fully, the cause of this was the diaphragms had become so hard and distorted in shape that they were maintaining the position of application, new diaphragms solved that issue , but someone’s answer was to back the adjusters right off !

Thanks again for the Info

Regards

Gordon

[Roy]

I’m not sure if you have self adjusting slack adjusters. If you do it is not recommended that you adjust your own, but it can be done and I have done it. Then I have my wife step on the brakes while I observe push rod travel on the brake chambers. As an alternative with the automatic slack adjusters backed off, I just left the air hose off the service brake fitting on the chamber and using shop air and a blow gun I gave repetitive shots of air to the brake chamber and watched to verify the automatic slack adjuster feature was in fact working.

Manual slack adjusters were simple to adjust so I never had to check travel because once adjusted they were right.

I used to change diaphragms, but often the flat plate that pushed on the diaphragm had enough surface rust that it wore into the diaphragm and shortened its life.