Author Topic: Mc8 won’t air up past 60 psi  (Read 726 times)

Offline SeanM

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Mc8 won’t air up past 60 psi
« on: November 02, 2019, 02:56:30 PM »
HELP  :'(.  Started off not being able to get air to register on gauge. Replaces brake chamber and lines, then two air bellows. Finally got it to build pressure.  However would only go to 60 psi. Looked around for more leaks...found one. On one of the tanks in the front of buss (driver side tank). Was able to temporarily plug it. Have a new tank on the way from US Coach. Still it will not air past 60. Getting frustrated on this thing. Anyone in Tampa that knows the system and can help would be greatly appreciated. Tired of chancing my tail on this.

Offline Ed Hackenbruch

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Re: Mc8 won’t air up past 60 psi
« Reply #1 on: November 02, 2019, 03:00:00 PM »
If you have a ping tank on the back bulkhead wall, check to see if it is plugged up.
Used to own a 1968 MCI 5A with 8V71 and Allison MT644 transmission.

Offline SeanM

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Re: Mc8 won’t air up past 60 psi
« Reply #2 on: November 02, 2019, 03:24:37 PM »
I’m able to add air to bus from the pink tank attachment but still doesn’t want to go past 60. And the ping take way takes forever. Is that normal? I’ll usually stay from ping tank then at about 30 I’ll turn bus on and use motor to fill quicker

Offline richard5933

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Re: Mc8 won’t air up past 60 psi
« Reply #3 on: November 02, 2019, 03:56:08 PM »
Stuck check valve? Air dryer stuck open? With the pressure stuck at 60, I'm wondering if there is a safety valve somewhere which is stuck?

Just stabbing in the dark here a bit, since my time has been spent on GM buses. If your manual shows the air flow circuit I'd start following from back to front.

On my bus there is an airline connection in the engine bay - that one will allow me to fill the entire system to 120 psi. There is also a connection under the driver's seat, but it doesn't fill the entire system. Perhaps you have multiple air connections?
Richard
1974 GMC P8M4108a-125 (Custom Coach "Land Cruiser")
1964 GM PD4106-2412 (Former Bus)
Located in beautiful Wisconsin
KD9GRB

Offline robertglines1

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Re: Mc8 won’t air up past 60 psi
« Reply #4 on: November 02, 2019, 04:33:54 PM »
common problems and solutions . air beams (built into bus frame one each side of rear and one across front) leak---solution use block off plates between top of air bags and air beams. You can do without the storage area/volume. with little noticeable ride differance = longer term problem solver.  Check to see if tank your talking about is leaking around straps.  PPV pressure priority valve-- Makes sure any pressure in system is directed to brakes first. Usually 45lb min.   Mix you up some soapy water( I like dawn) in a old spray bottle.  Make sure check valve between wet and primary tank works. Like you say it would be nice to get a hands on opinion/help. I had a mci 8 at one time. Also check tank drains (pedcocks) known to leak - Cheap to replace. in rear area just forward of step up(inside floor) there is a access engine cover--Air compressor is accessable to check for leaks there. From what I remember and not ...
Bob@Judy  98 XLE prevost with 3 slides --Home done---last one! SW INdiana

Offline Fred Mc

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Re: Mc8 won’t air up past 60 psi
« Reply #5 on: November 02, 2019, 05:11:33 PM »
I would think that a leak large enough to prevent pressure above 60lbs would be VERY evident. Perhaps its the pump that is worn or the regulator. When mine went it wouldn't air up at all.
Regards
Fred

Offline richard5933

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Re: Mc8 won’t air up past 60 psi
« Reply #6 on: November 02, 2019, 05:44:36 PM »
Has it ever gone over 60 since you owned it?
Richard
1974 GMC P8M4108a-125 (Custom Coach "Land Cruiser")
1964 GM PD4106-2412 (Former Bus)
Located in beautiful Wisconsin
KD9GRB

Offline SeanM

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Re: Mc8 won’t air up past 60 psi
« Reply #7 on: November 02, 2019, 07:47:28 PM »
I have only owned it for 4 weeks. But no, hasn’t been up at all. Just recently got it to 60. Been trying to get it off the lot I bought it from since. The biggest leak found what on the tank. Haven’t come across anything else. I agree though preventing it from going above 60 would be i big leak

Offline buswarrior

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Re: Mc8 won’t air up past 60 psi
« Reply #8 on: November 02, 2019, 08:32:50 PM »
briefly, think bad things of previous owner, then accept that as what it is, and move on...

reaching 60 lbs suggests some good things as far as the the core brake tanks, not a guarantee, but at least you got this far, the pressure protection valve opens around 60 lbs, so all the auxiliary stuff now gets exposed, suspension, wipers, shutters, fan dampers, belt tensioners all combine to destroy further air building efforts...

Do you have a gun to your head to get out of there?

You have to solve each source of leak, as you find them, until you get some sort of air integrity.

Go back, go back again, go back until you SCREAM, and find the leaks, fix them, and it will drive out of there.

It is some awful big leaks to defeat the engine driven air compressor, find the big ones, work backwards from those.

happy coaching!
buswarrior



Frozen North, Greater Toronto Area

Offline chessie4905

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Re: Mc8 won’t air up past 60 psi
« Reply #9 on: November 03, 2019, 04:04:42 AM »
If you haven't done already, you need an air source that doesn't  make noise to drown out the leaks. Move aux. compressor away with long hose, and around a corner, place in vehicle, whatever to eliminate it's  noise.
GMC h8h 649#028 (4905)
Pennsylvania-central

Offline robertglines1

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Re: Mc8 won’t air up past 60 psi
« Reply #10 on: November 03, 2019, 06:29:50 PM »
does bus rise to drive height? About 12 inches below bottom of Bay doors.  Is gauge any good?  I know your passionate about buses or you wouldn't be trying so hard.  lot of experience on board.. will help as long as you ask questions.
Bob@Judy  98 XLE prevost with 3 slides --Home done---last one! SW INdiana

Offline SeanM

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Re: Mc8 won’t air up past 60 psi
« Reply #11 on: November 04, 2019, 08:57:08 AM »
I plan on trying to move air source further away to see if I can hear any leaks. Also will be using lots of dawn and water.
Bus does not go to ride night. Gauge does seem to work. Can watch it go to 60 psi then seems to stay. This is even with motor compressor. I’ve check all tanks to see if air in them and they do get air.

I have definitely thought bad things of the previous owners. This might even be why they got rid of the bus. I’m trying to be calm and rational with it but I’m loosing patience. If I could just get it to when I will be keeping it I wouldn’t be stressing so much. It’s the time schedule of it being stuck at place of purchase. Tried to have it towed. No one is willing due to air suspension not working.

Offline SeanM

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Re: Mc8 won’t air up past 60 psi
« Reply #12 on: November 04, 2019, 08:59:19 AM »
I do have maintenance manual. However the air line charts are all blurred so can’t understand what is what. Having hard time figuring out what is what and what goes where to even check valves.

Offline Astro

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Re: Mc8 won’t air up past 60 psi
« Reply #13 on: November 04, 2019, 09:06:17 AM »
If you bought it, order up a landoll and take it home and park it.  Relax. Then get to fixing it on your terms. $750 is cheap considering you just adopted a bus.
Ken
Arlington, WA
1971 MC-5B, U7017, S9226 (On the road)
1945 Flxible Clipper (In conversion)
1945 Flxible Clipper town buggy

Offline robertglines1

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Re: Mc8 won’t air up past 60 psi
« Reply #14 on: November 04, 2019, 09:18:05 AM »
watch gauge when it hits maximun (60 lbs you said) push brakes and hold.don't pump. If the air basically drops out of site it's on brake side.... little change say 10 lbs= look to accessory side( air bags ) etc 
Bob@Judy  98 XLE prevost with 3 slides --Home done---last one! SW INdiana