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Completed Jon’s air system test – now??
I completed the Norgrens today – did not have the guts to test the system – yet.
And before I started the brake chambers I ran thru Jon’s tair tank drain down test. And found a surprize.
I ran the test both directions so to speak.
Reverse direction:
1) Drained aux tank, primary and secondary stayed at pressure.
2) drained the secondary – both secondary and primary dropped together
3) drained wet tank which still had air.
Forward direction:
1) drained wet tank, primary and secondary and aux stayed at pressure
2) drained Primary, Primary and secondary dropped toghether, Aux stayed at pressure.
3) drained aux tank.
For longer term durations my aux will leak from 100 to say 60 overnight, the primary and secondcony always leak down together and will drop to 60 psi over a couple of days often holding around 50 psi for a week or more.
The good news is no water/oil in any tank.
Question 1) I know this is not correct. What do I change? or what are my next trouble shooting step? If these are in the maint. manual just send me to the correct section.
Question 2) as I started to loosen the flexible air line at the brake chanbers most (maybe not all six) were trying to twist the air line and not turning freely. Is there a way to free those? Should they be replaced with the chambers? I suspect they are original – so 20 years old. They look good except for a lot of rust on the fitting and spring at the lower brake chanber end especially the tag axle.
As always many thanks
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30 Replies
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Joe,
The primary and secondary (rear and front) tanks should not have drained simultaneously so the check valve (I think it is called a double check, but verify that) is not functioning. The intent was to be able to have a loss of air in one brake system, but retain pressure in the other.
I think if you can pull up the pneumatic system diagram fo ryour vintage coach it will show where that valve is located. I believe the line from the wet tank goes to the primary and tees off to go to the secondary so they both fill at the same time. Only the aux tank has a protection valve so I believe you are dealing with check valves only.
If I remember correctly the service brake hose fitting at the brake chambers is a swivel with a male pipe thread adapter threaded into the chambers. If the fittings are not rotating freely and you have rust you may wish to change all rubber service brake lines. They do fail due to age even though they look OK. Any hose shop can make hoses that are exact duplicates of the ones you have or I am sure Prevost has them in stock.
Generally speaking if the brake systems are losing pressure when you are parked the places I would look for leaks should be the air tank petcocks, the foot treadle, or the relay valves. Those are the places that see pressure when parked. There is no pressure at the brake chambers when parked. Typically your brake system pressures should hold for months.
The aux system pressure loss is a whole different situation. With so many systems and devices that have air pressure on them when parked it is probable there is not one single location where pressure is lost, but perhaps a little air loss in multiple places. Common places for the XL are at the regulator for the belt tensioners, at the regulator and distribution block behind the passenger side tag axle, at the Norgrens (not likely now) or their fittings, generator air bags, and many other places that Prevost or the converter had aux air.
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Thanks Jon,
I’ll get a double check, and a set of hoses.
I’ll tackle the aux system later. I’m taking the 3 year plan to zero the coach. Fluids, house and Chasis batteries last year, Norgrens, brake chambers now and 4 tires later this summer. Rest of the tires, air bags and shocks next year. I should be close.
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The park brake valve will leak down both gauges at same time , check replace valve and fittings with comp fittings. If it leaks down in couple days it’s a place to start. The tag valve will leak down the aux air
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Peter et al, if the parking brake leaks down when parked something is wrong because there is zero pressure to any brake chamber when parked. When the brake is released the parking brake receives air pressure to compress the spring.
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Jon what I was trying to say is if u undo both lines on park brake valve at dash it will leak both gauges down , the park brake valve is a easy place to start. I do not have air drawing with me but I think one line goes to the SR1 valve and the other goes to the R14 valve either one of those valves can also leak both gauges down together because the shuttle cock valve works off of which ever tank has the most pressure in it , in case of lose of air in the primary tank it will use the secondary tank air to make brake applications in a emergency. That was the way it was on my 1997 Royale it may be different on other years of coaches. Sorry if I was not clear about it
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Peter is correct, the parking brake valve is one of valves that is fed by both primary and secondary air systems. There are very few items that do. If you can get to your parking brake valve they are really inexpensive to replace.
The details on air drawing for late model XL coaches isn’t much help. There is no legend and it’s nearly impossible to see things like check valves in the drawing. The illustrated parts breakdown gives great detail. I recommend they be used together, at least on late model XL coaches.
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Peter,
I understand the tag raise system to work this way. The aux tank supplies the Tag Control valve which is normally open. This keeps air pressure on the pilots for the (3) port values which controls the air to the tag air bags and the lift cylinders. When you raise the tag you actually cut off the air to the pilot and the spring forces the norgren to the side to release pressure from the bags and the put air on the lift cylinders. This is also the famous system which release the pressure on the tag bags when the aux pressure gets to about 35 psi as the air pressure can no longer over come the spring.
So when you say “The tag valve will leak down the aux air” its not that the valve leaks thru but that the fittings to the valve leaks as they always have pressure on them. Correct?
On the other hand the parking brake valve gets it’s supply from the double check, which means its supplied from both the primary and secondary tanks. When driving that valve is open to hold the spring back and release the parking brake. When parked that valve is closed. So if either the fitting to the valve leaks, or the valve leaks thru then both the primary and secondary will leak down together over time. Correct?
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I would say you nailed it Joe.
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Really interesting thread. During my bus renovation I removed the driver’s seat and thus disconnected the ISRI air line. I noticed thereafter that it was not possible for the bus with engine in hi-idle to reach full air pressure. I would think there is some sort of safety to prevent this. Do I have a check-valve that is bad?
Thanks!
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There could be a lot of reasons for failure to reach full pressure ranging from an out of adjustment governor to a big leak.
A leak so large it will not reach full pressure (meaning the compressor is not cycling but running continuously) will be clearly heard and within a short period of time system air pressure can go to zero. The gauges will help define the problem. What is happening and what do the gauges do immediately after you shut down?
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Sorry, I should have stated that after re-installing the seat I had a fully-functional, normal system and no issues on a long trip to Lake George.
With the seat out, and the bus running the engine compressor got the guages within 5 pounds of normal. It was just clear that the amount of air leaking from the disconnected ISRI seat line was significan volume that the bus couldn’t keep up. This could be totally normal but I thought I remembered reading a few years ago that the aux systems were isolated from the braking systems for safety reasons, so it surprised me.
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Thanks Jon,
Worked 6 hours today – removed 5 nuts and 3 supply hoses from the tag brake chambers and one of the front. The nuts are so crusted in stuff I’m having a devil of time getting them to break free, even with liberal use of PB blaser. Thank goodness for a stong compressor and a 1/2 impact wrench. I actually sheared 2 of the studs rather than getting the nuts off.
Any hints on how to get the lower nut off the front chamber? I can’t get a socket on it since it is blocked by the slack adjuster arm. I had a porta-power putting force on a closed end wrench and all I did was flex the wrench handle! I would have gone for the red wrench but there is no torch in my stable.
I doubt if these have ever been changed.
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I too just replaced brake chambers this spring on drive and tag, and I think all the nuts were loosened with the half inch impact. Maybe you can get on it with a universal joint, seems like I did have to use it. But didn’t destroy any thing in getting the old ones off. That pb blaster needs to soak for a day to realize its full potential, good luck.
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Sorry Joe was doing clinic today but you got it right
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The aux system is isolated from the brake systems by the protection valve which will not allow air to enter the aux system until the brake systems have been satisfied with around 75 PSI. That is one of the checks made when airing up the bus to verify that is the case.
Joe,
Can you get a nut splitter on the stubborn nut? Can you get a die grinder with an abrasive wheel at the nut? If you want more room adjust the slack adjuster arm so it rotates out of the way.
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