[Archived]

Have you pressed brake pedal with park brake “off” to see what your air pressure gauges say.

[Archived]

I would urge you to do a DOT brake check. It sounds as if you have a leak in the parking brake circuit.

If you do not know how to do a brake check, at least do the following:

Use chocks or some means of preventing the bus from rolling. start the engine and build up air pressure. Turn off the engine and release the parking brake. The brake system(s) air pressure should drop a little and then stabilize. If it continues to drop the problem is associated with the parking brake, either a failing diaphragm in a brake chamber or a leak in one of the hoses.

[Fred F.]

When the coach is stationary with the park brake on or off all 3 gauges are steady & no compressor cycling or purging. Engine off gauges are steady 120, 120 & 110. Driving the primary & secondary gauges remain steady at 120psi. Only the acc tank gauge cycles between 100psi to 125psi 1 time every minute. No brakes applied, same with trans retarder on or off, etc…no specific correlation to a vehicle controls perspective.

[Fred F.]

I want to make sure I understand this. With the engine off, and the parking brake released no loss of air in any of the three systems, but only when you have the engine running you lose accessory pressure?

Assuming I have that correct, try starting the coach, building air pressure, and then going for a drive. I assume the level low selector (coach leveling system selector) is in the road or driving position. I am also assuming that is when you see the aux (accessory) pressure cycling up and down.

If that is the case, make sure the three switches Liberty installs to raise or lower the front left or right are all in the neutral position and then switch the main selector switch out of the road position to the manual position.

If I understand what you are saying it sounds like the issue has to do with driving, but only on the accessory system. That seems to isolate the problem to the suspension because I cannot think of any other part of the pneumatic system that is only active with the key on and while driving. If it does not cycle with the selector in the manual position my focus would be on the part of the system associated with the ride height valves and ride height air lines and possibly a 5 port Norgren.

A leak as large as described is going to be audible.

[Archived]

Roger when I 1st got my coach it did the same thing, driving by houses slow I could hear dryer purging, or on fast idle it did the same thing, drove me nuts, so I ended up replacing the valves in the head of the Tu-flo 750 compressor the unloader valves, after I did that it stopped doing purging. sitting there on fast idle it does not purge, you can pump brakes and then pressure comes up and purges. I think my dryer had a bad check valve in it also. looking at air schematic the compressor builds air to dryer to wet tank, tee off to primary and to secondary tanks and before secondary tees again to aux tank, the primary and secondary tanks have check valves to hold in air for protection, but over 70 psi if wet tank drops from compressor the aux will also drop. If you still have the original dryer and check valve is not holding air in to wet tank and wet tank is monitored from small line to the governor. And if compressor unloader valve are not holding air leaks from wet tank and there is problem.

[Archived]

Jon,

Yes on your first statement but engine off or on all 3 gauges are stable.

While driving the 1 min compressor cycling seems to be independent to most all air accessories like, trans retarder on or off, pass floor open or closed, air door lock on or off, air seat on or closed off. Switching level low from road to manual w all corners in neutral remains the same 1 min purge. Except there will be a slight negative 1psi glitch when in the road position & large quick dips in the road surface. The bus will stand proud for a couple weeks before dropping 1 in in curb side rear. All Norgrens are new. Definitely should be audible, but only while driving. I think i clarified all your comments. Thanks Jon

Art,

I agree w your description of the air routing & similarity of purging cycle while driving. But it does not purge every min stationary in high idle. I can both hear the purge & see that on the acc gauge while driving slow. The ss1200 dyer has been replaced. Both primary & secondary tank check valves are replaced & PR2 on acc tank. Also the protection valve in the steering comp. The compressor is original w 107,000 miles on it. Good point.

Thanks

[Roger]

I replaced unloader valves at 118,000 miles but its also 21 plus years old! did you say on high idle is that every 20 to 30 minutes? mine would cycle on fast idle about every 5 or so minutes

[David]

At high idle it is over 20 minutes. Probably longer now as many small leaks have been corrected recently. Primary & secondary gauge now drop approx 10psi over night or 12 hours. Can the truflo 750 valve replacement kit be performed in frame?

[Roger]

I’m trying to get my head around this. If I have the facts correct this is puzzling.

With the bus systems air up, engine on or off, but the bus stationary, parking brake on or off pressures remain constant and the compressor does not cycle every minute.

But as soon as it is put in gear and is in motion the compressor one minute cycling occurs, and it is the accessory system with a pressure drop that appears to trigger the compressor cycling.

Is that right?

Now I am going to go crazy trying to figure out what changes from the coach stationary in idle or off, to one in gear and rolling.

[Ronald]

Suspension, ride height valves adding/dumping air as the coach moves would be a logical source for the problem and you have 3 of them.

[Roger]

Jon, you got it!

Tom, switching level low from road to manual locked or all 3 corners in neutral…very small psi drops can be seen when going over bumps in road position. No little psi drops in manual locked. Looking for 15psi per min steady drop in acc (wet tank) air pressure. Primary & secondary brake air supply remain rock solid at 120 psi during all this.

[Tim]

Tom, pay attention. I thought the same thing and Roger tried to see if that would make a difference, but no joy.

This is going to be one of those problems that makes no sense. We are missing something.

When the coach is being driven it seems the only difference between it sitting still is the parking brakes is off, but Roger ruled that out, and then has ruled out ride height valves. WTF??????

I think Roger needs a volunteer to run alongside the coach while it is in motion to listen for and locate the air leak that has to be substantial to cause once a minute compressor cycling. What does not make any sense is for the air to leak down that fast, my guess would be it would take forever for the compressor to build pressure to the cut off pressure, so now I am trying to puzzle through what part of the compressor / dryer system would act that way. But I still cannot get my brain to get that to tie in with having the coach in motion.

Roger…….anything else you can add to our understanding of what is going on?

[Roger]

When the comp cycles it takes approx 5-10 sec to reach cut off 125 psi. Then 1 min to drop to 100 psi & start over. The primary & secondary remain at 120 psi. Under braking, if all tanks must be filled the charge time increases to 20-30 sec depending on engine speed. At times the cycle rate gets worse, every 30 sec. At times it seems worse under acceleration?

[Tim]

An air suspension drivers seat, if installed, could possibly contribute to the mystery. I don’t turn on my air seat until I am ready to move the coach. Just another possible contributing factor if the test drive is on a bumpy road.

[Fred F.]

I have a ball valve to isolate the seat but have not tried it yet. I have bounced up & down like a jack rabbit in the seat & it has no effect.