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Author Topic: Bus won't air up  (Read 2672 times)
rip
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« on: April 21, 2008, 11:04:33 AM »

I have an 85 Prevost with an 8v92 and I can not get the bus to air up.I have been sitting since Nov and today I started the up to change all my or bags and the bus didn't go up at all. I have an external air compressor which I can hook up to the air system and that did not do anything.It ran for about five minutes and shut off like it was full of air.Any suggestions? I ran the bus back and forth a few times with no change. When I was by the engine I thought I heard a lot of air leaking.I found air leaking out one the valves with a plunger on it but I do not know if that is normal if it would cause the bus not to air up.I can't hear and air leaking anywhere else,but the bus is way to low to get under it.Thanks for your help in advanced.
Don
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niles500
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« Reply #1 on: April 21, 2008, 01:00:41 PM »

Just a reminder - do NOT get under the bus until it is properly cribbed - I'm sure you know but I'm just making sure - HTH
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Sam 4106
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« Reply #2 on: April 21, 2008, 01:11:46 PM »

Hi Don,
It sounds like the purge valve on your air drier is stuck open. If the leaking valve is on the bottom of a cylinder with a cone shaped bottom, tap gently on the valve shaft to close the purge valve. If the valve closes you will probably build air.
Good luck, Sam 4106
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1976 MCI-8TA with 8V92 DDEC II and Allison HT740
rip
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« Reply #3 on: April 22, 2008, 05:15:51 AM »

I checked the bottom of my air dryer and found the plunger hanging down. I'm assuming the plunger needs to go up but it will not budge. Any suggestions on how to fix this?I'm getting ready to leave Phx. for the summer,but I'm not going anywhere until I solve this problem. Thanks
Don
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ChuckMC8
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« Reply #4 on: April 22, 2008, 08:04:30 AM »

Don, you need to service your air dryer before you leave. There's instructions for doing the job on the Bendix web site ( www.bendix.com ) You need a desiccant catrrtridge and a purge valve kit from Mohawk or MCI to properly do the job. Its not difficult but on most buses it has been neglected.
   Or else you will have t r o u b l e and it will put you on the side of the road.
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« Reply #5 on: April 22, 2008, 08:21:26 AM »

Hi Don,
Did you have the bus running when you looked at the air drier to see if that is where the air is leaking? If not, I would do that simple step to be sure that is the problem. If that is the problem, I would remove the air drier, take it to a truck parts department, and ask the price difference between a rebuilt and a rebuild kit. Rebuilt driers are sometimes priced very near the cost of the rebuild kit. I would caution that you clearly mark the air lines so they get put back in their proper locations. Please let us know the outcome.
Good luck, Sam 4106
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« Reply #6 on: April 22, 2008, 01:30:50 PM »

I had a similar problem with my bus. It would not air up at all. The air pressure guage never left zero psi. My problem was with the air dryer. On the very bottom of it is a little rubber boot with a small hole and little slices cut into it. Air was spitting out of it. There is bolt, thinking 1/2" or 9/16" up inside that rubber boot. I turned the bolt about a 1/4 turn and the air quit spitting out and the bus started lifting right up. I watched the air system and made sure it was working properly after that. That was a couple months ago and it is still airs up and maintains air pressure.
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« Reply #7 on: April 22, 2008, 01:54:33 PM »

Can you post a pic of this valve?
Bendix makes replacement parts for their air dryers too.
Truck parts house should stock them also.
Check their website, might help you identify this part.
http://www.bendix.com/en-us/products/airtreatment/Pages/default.aspx
« Last Edit: April 22, 2008, 01:56:45 PM by Sammy » Logged
rip
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« Reply #8 on: April 23, 2008, 05:53:28 AM »

I finally found the problems. I checked the air dryer and no air was coming our. I found an air valve that controles the throttle was leaking around the plunger big time.After plugging that,I checked the air dryer again and found  a lot of air coming out the bottom.I got the parts out and waiting to go to Phx. to get the parts.I'm still building my house so I have been really busy.
Thanks for the help.
Don
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rip
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« Reply #9 on: April 25, 2008, 11:21:13 AM »

After all that work the bus still won't air up. I replaced the air dryer with a rebuilt and still the same results. I still am getting some air out of the bottom of the air dryer.After running the bus for a while,the plunger of the drying is down.Is that normal?The other change is my external air compressor will not shut off when hooked up to the bus.Also,both of my air gauges of the dash read 120psi.What else can I check?
Don
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Sam 4106
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« Reply #10 on: April 25, 2008, 12:20:04 PM »

Hi Don,
Now you have me puzzled. Since the dash gagues are reading 120 PSI I'd say that your bus is aired up. In your original post you said you moved the bus so I'm guessing the parking brake released. If so the brake system is aired up. Do the service brakes work? When you say the bus won't air up, do you mean the air bags still aren't filling? If just the suspension is the problem, my guess would be that the valve that lets air into the tank for the suspension is stuck closed. I let the guy that bought our Prevost have the manuals so I can't look up where that valve is located. Maybe Sammy can help with that.

The air drier on the '84 Prevost we had would exhaust air from the purge valve when I aired up from shop air since my shop system was set at 150 PSI. Is your external air compressor pressure switch set for more than 120 PSI? If so, that could be  what is causing your rebuilt air drier to leak and if your external air compressor doesn't pump enough volume to overcome the leak it will not shut off.

Good luck, Sam 4106
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rip
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« Reply #11 on: April 25, 2008, 06:59:05 PM »

Your right.The air bags
are not airing up. The parking break does release and the service brakes do work.The air compressor does not build up more than 120psi. Does anyone know where the valve is that Sam is talking about?
Don
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niles500
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« Reply #12 on: April 25, 2008, 11:17:34 PM »

First things first - replace your air governor - HTH
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Sam 4106
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« Reply #13 on: April 26, 2008, 08:57:04 AM »

Hi Niles and Don,
I'm believe the bus air system is airing up but the suspension is not. However, Don hasn't specifically said if the bus air compressor is pumping or if all the pressure is coming from the external air compressor. Don, would you clarify if your bus compressor is working? I think the valve I am refering to is the pressure protection valve and may be located on or near one of the air tanks. I don't know which tank though.

One thing you could try to free that valve is to completely drain the air system. You may have to drain the brake air by repeatedly applying the service brakes until your dash gagues read zero. My Prevost would hold brake system air for months and it doesn't sound like you want to wait months. Then refill the air from your external compressor at the tank under the drivers area by the steering secter. That will put reverse pressure on the pressure protection valve and may force it open if that is where the problem is. If the dash gagues show pressure when you fill the system from the front tank then I think the pressure protection valve is open. Also, your suspension may come up by applying air to the front tank.

None of my ideas may work, so please remember that I am trying to help and it isn't as easy as being there with you.

Good luck, Sam 4106
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1976 MCI-8TA with 8V92 DDEC II and Allison HT740
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« Reply #14 on: April 26, 2008, 12:10:18 PM »

Yes,the bus air compressor working and was just replaced a few years ago at Luke's. I will try what you said tomorrow or Monday.
Don
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