-
Cycling Air Gauges
I drove 650 miles three weeks ago from Arizona to California and was parked two weeks. All was normal when I cranked up last Monday, air pressure was good and the brake check was fine. It took me about 3 miles to get on the interstate and once I was on the interstate I noticed my air gauges were cycling some.
I timed the cycles for over an hour and this is what it was doing.
The top gauge, which I believe is the secondary gauge, was taking about 4 1/2 to 6 1/2 minutes to go down to 100. Once it reached 100 the compressor kicked on and it cycled back up very quickly. During that time the primary gauge would go down to maybe 115.
Then the primary gauge would go down to 100 in about 4 to 6 minutes. Once it reached 100 the compressor would not kick on until the secondary gauge went down to 100, 2 to 3 minutes later.
A few times while waiting for the secondary gauge to get down to 100, the primary gauge hit about 95 or a little higher. Once the secondary gauge reached 100 the compressor kicked on and it cycled back up real quickly.
(One note: My secondary gauge always seems to be about 5 pounds higher than my primary gauge. I have always suspected that’s just a little anomaly in the gauges and the pressures are probably the same. I am giving you the numbers that I actually saw.)
This was a very consistent pattern for about an hour and 20 minutes. The secondary gauge would go down to 100 and then cycle up. Then the primary gauge would go down to 100 or slightly befow while the secondary gauge moseyed down to 100 and then both would cycle quickly back up. Then it would repeat. Like I said, I timed this consistently for over one hour and 20 minutes.I was on a very busy section of Hwy 99 from Merced down through Fresno and drove until I reached the Tulare rest area. I stopped at the rest area and first checked all my wheel temperatures. They were all within 10°.
I pumped my brakes until the compressor kicked on and both sides held 115 to 125 going on 15 minutes while idling and the parking brake on. In hindsight I should have chocked the wheels and released the parking brake and seen if the pressures held then.
With the bus running and the parking brake off, we held the brake pedal down and the pressures dropped about 5 to 10 pounds and then held losing nothing noticeable that we could see over three minutes.
With a bus off and the parking brake off, we pressed the brake pedal and it dropped 10 pounds or so and then held losing nothing noticeable over three minutes.The rest area was very noisy with highway traffic but I could hear no air escaping with the brakes held down either running or not running.
I touched base with Jeff. (I am so glad and thankful he still takes my calls and emails.) I did not know if I should detour to Mira Loma or press on to my destination in Ft. Worth. It was a moot point because the problem turned out to be intermittent and was completly gone after that. I drove 1600 more miles over the next three days with no problems and no cycling of the gauges at all.
I am tied up with the tent this week but I could try to get in to Prevost Ft. Worth early next week. I am very hesitant to take it in with a problem that is not manifesting itself at the moment and turn it over to them along with my revolving credit account. I would do that with Jeff but very hesitant otherwise.
Long time PC members may remember I drove this bus for 4 years with contantly cycling air gauges. I know better now but I thought it was normal. The guy that sold me the bus said it was perfectly normal and they all do that. He was a salesman after all and not an honest one either. That whole ordeal came to a halt with a failed brake chamber along the interstate on a very cold and windy Oklahoma day in February.
I do not wish to repeat that. Obviously whatever was leaking last Monday, was not leaking after I stopped. Help me here. Any ideas on this distinct pattern of cycling? Anything that could be temporary? Something not seated properly?
Thanks in advance.
Davy
-
33 Replies
-
Davy,
This is one in which I would like to have Peter participate because he is really up to speed on the pneumatic system, especially the brakes. My gut in reading your description kept telling me a relay valve on the emergency brake side is at the heart of this.
I feel confident in saying it is not a brake chamber or fitting or air line because they don’t repair themselves or have intermittent leaks. If your air loss is in any of those items you are still going to have the leak.
I do not think it is a service brake relay valve because that would require the primary and the secondary to alternate leaks and then to fix themselves. Actually a relay valve leak is because one is just not seating correctly or has some foreign matter causing the leak. I would suggest as much as you rely on the coach to just reploace all four of the relay valves, both R14 and R12. They are pretty cheap ($60 range) and easy to access.
-
Davy,
Sounds like what I had going on a couple years ago coming into KC, except my cycle was down to about 2 minutes. Once I parked you could hear the air leak on the air brake controller. I replaced it in KC with a rebuilt one and just replaced that with a new one this month as the rebuilt was showing a VERY slight leak when we checked it.
Not saying that is what it is, but the symptoms where exactly as you described. Lucky the leak wasnt so bad that the onboard compressor couldnt keep up with it. Not sure I would have gone much further then I did (about 100 miles total).
-
Do you mean the foot treadle Charles?
-
Jon, my issue was the Bendix r14 valve.
-
Yes, that is what I posted above and I cannot think of any alternative to Davy replacing all of them because any one can begin having issues at this point due to their presumed age.
-
It is most likely the R 14 relay valve , the park brake valve could also be leaking, but if the threeR12 and R 14 have not been replaced I would do all four of them you can rebuild them but it’s about the same amount of work plus you can then put comp fittings on all the air lines, if you put new relay valves
-
Thanks, guys. I had one R14 or R12 replaced a couple of years ago in Houston. The rest of them are unchanged, I believe. I was there to have a leveling valved changed on one side and when I pulled out, the bus would not hold pressure. The replacement valve took care of it. That was before I had so much more replaced at Jeff’s.
Next, question. Is this something I should try to get into Ft. Worth next week (I have no idea if they can get me in and I am very pressed for time, but I know you make time for emergencies) or Jeff’s later in the year?
Oh, to be in Tennessee!
Davy
-
Davy,
Any good truck repair shop can do the job if you cannot get into Prevost. That relay is generic and common on air brake systems. The only issue might be bolt mounting holes might vary, but that can easily be dealt with if necessary.
-
Davy,
From my experience with replacing one on the road, this is a common valve found on many trucks (I want to say the mechanic that swapped mine out actually mentioned trailer brakes). The main thing you need to deal with is paying attention to which ports are blocked and which are used for what lines (Prevost blocked off one port I think). There is a bracket that attaches to the valve body using standard holes on the valve and then attaches to the chassis via the bracket, so nothing custom really needs to be done to make it work.
-
This is a tip in case anyone has a relay valve fail, and you cannot build coach air pressure.
First, understand if you are not in a safe place you can over power your brakes even if the spring brakes have applied. Drive only long enough to get further off the road or a little out of the way. Your bakes are going to get very hot so if you have to do this remember the greater the distance the hotter things are going to get unless you pause to let things cool. I am talking about walking speed or less.
With the engine running, but air not building you cannot build pressure or restore pressure to your aux air system. To replace the relay valves someone has to get beneath the coach and your alternative is to try to jack it at the support points, lift it with a tow truck ($$$$$$$$$$$$) or raise it with air bags for access.
Run your generator and aux compressor. If you have a lower pressure selected, adjust the aux compressor pressure at the pressure control switch. If you don’t know how to do that, learn how and write it down so if you do have to have higher air pressure you will be able to do it.
Get the aux pressure up as high as reasonable. Some compressors have a 100 PSI limit, but that should be OK. With the key on manually raise the coach in the area you hear the relay valve leaking using your level low controls. Some coaches will not lift the front with just 100 PSI, but you can nurse it higher by simultaneously lifting the tag axle.
Once you get the coach high enough so someone can get beneath it, block under the chassis support points for safety. Once someone slides under the coach that person can sit up in the wheel / axle areas and have room to work to remove and replace the R12 or R14 valves.
If you are near any air hose, or have a service truck come to the coach the Schrader valve in the steer compartment will be how to pressurize the aux or accessory air system to raise the coach if the aux compressor will not do the job.
I am not sure what is going on but there have been a number of reported issues with relay valves in the recent past so be mentally prepared to do what is necessary if you experience a problem with one. They are one of the very reliable parts on our coaches, but if one does fail it can put you in a world of hurt. Do not let any service company give you a line of BS about how expensive they are or how hard to replace. It is an easy fix in the field, and safe if the coach is blocked up. If you have any concern about the relay valve condition in your coach it is not that expensive to swap out the old ones for new.
-
Replace them all ASAP. They are cheap compared to a wrecker comming out ($2,5000 +/-)
If the R14 goes you wont be going anywhere. I think that is the one I replaced.
Also, someone correct me if I am wrong but there are several variances to these valves. Hose locations and such.
I bought a whole bunch of R14s from a friend that furnishes brake parts to the local trucking industry and they would not fit.
If you can verify with Prevost. They aren’t costly.
I think my parts said an R14 Vertical. (the way it mounted I guess)
-
My records from Oct. 2013 say Prevost Houston replaced two R-12 valves. I remembered them only replacing one but the records say two. The parts were reasonable and the work was quick. The labor from the leveling valve the day before was horrendous, but lesson learned.
I will contact Prevost Ft. Worth and see if they can squeeze me in Tuesday to replace any R-14 and R-12 valves that have not already been replaced.
We hope to get the tent down dry Sunday evening and I had planned to hide and sleep until I need to be south of Houston Wednesday. So much for that.
Davy
-
Historically our coaches have used Bendix brake pneumatic system components so if you are buying replacement parts try to get Bendix.
In a pinch however they are generic as to their function, but mounting holes and configurations may vary.
-
Davy, it could very well be the singnal air line going from the air compressor to the governor . Peter………
-
My vintage coach uses R-6 relay valves. So I don’t have experience with teh R12 or R-14. However, I can say R-6 is generic and Bendix has about 30 sub part numbers with different mounts and port configurations. If at all possible I’d urge you to buy them directly from Prevost. If it’s like mine, having the exact replacement will save you significant time and expense. Prevost was back ordered 180 days so I picked up a couple at NAPA – same part number but it was a completly different valve – I have to return them. I ended up getting my replacement from Luke at US Coach. He had it on the shelf. I ordered the other replacements for the rest of the coach diretly form Prevost. It’s been months STILL waiting……….
I opened up the failed valve and did not see anything unusal (no root cause of failure) I will say it has some rubber seals in it. So, like everyintg else on these coaches – if it has rubber in it, on, it or whatever – it will age out sooner or later. I’m adding relay valves to my list of things to keep an eye on and will likely replace them on a frequency yet to be determined.
Log in to reply.