Author Topic: Mc8 won’t air up past 60 psi  (Read 1464 times)

Offline buswarrior

  • Hero Member
  • *****
  • Posts: 5791
  • '75 MC8 8V71 HT740
Re: Mc8 won’t air up past 60 psi
« Reply #15 on: November 04, 2019, 09:27:56 AM »
You have a very large leak, it will not be noisy, big leaks just let air out.

You need a collection of fittings in order to put an air fitting and a quarter turn valve into the drains on your air tanks.

Put air directly into tank under the driver, accessible through the door outside under the driver window, under the electric panel. And report back on what that does for you.

You could have tanks rusted out, burst/aged hoses, valve internals leaking out exhausts.

Go and turn the air supply to the shutters/dampers right off to get that ball of mess out of the picture for now.

You are going to need air schematics, better work on getting that sorted out to. No reference materials, can't help yourself.

Happy coaching!
Buswarrior
Frozen North, Greater Toronto Area

Offline Jim Eh.

  • Hero Member
  • *****
  • Posts: 919
  • Excess in moderation
Re: Mc8 won’t air up past 60 psi
« Reply #16 on: November 04, 2019, 12:46:18 PM »
My suggestion would be to remove the compressor discharge line AT the compressor and connect an alternate air source. It may take a few fittings to rig up but since it is a difficult leak to find, start at the source and work your way down stream.

I have had main air lines with holes in them the size of Seattle. It is something that you would think would be immediately obvious but like said above, large holes = little sound. You may even if you have to plug the other end of the main line to find a leak.

The system normally would not leak back out the compressor discharge line when charging the "wet" tank due to a check valve on the inlet of the tank. The check valve may be missing or inoperative between the tank and the compressor line.
"Some days it's just not worth chewing through the restraints"
Jim Eh.
1996 MC12
6V92TA / HT741D
Winnipeg, MB.

Offline SeanM

  • Newbie
  • *
  • Posts: 33
Re: Mc8 won’t air up past 60 psi
« Reply #17 on: November 04, 2019, 01:31:23 PM »
Well finally getting her home. Found someone to tow her

Offline SeanM

  • Newbie
  • *
  • Posts: 33
Re: Mc8 won’t air up past 60 psi
« Reply #18 on: November 04, 2019, 01:32:32 PM »
Will now have ample time to diagnose her sickness. I will still be asking for help lol

Online richard5933

  • Hero Member
  • *****
  • Posts: 2701
Re: Mc8 won’t air up past 60 psi
« Reply #19 on: November 04, 2019, 01:48:20 PM »
You're way ahead of many starting out. You know the engine runs and you'll soon have it at home so you can work on it at your leisure. Good news on the tow.
Richard
1974 GMC P8M4108a-125 (Custom Coach "Land Cruiser")
1964 GM PD4106-2412 (Former Bus)
Located in beautiful Wisconsin
KD9GRB

Offline buswarrior

  • Hero Member
  • *****
  • Posts: 5791
  • '75 MC8 8V71 HT740
Re: Mc8 won’t air up past 60 psi
« Reply #20 on: November 04, 2019, 04:03:22 PM »
Reaching 60lbs and no further suggests the leak is on the aux side. If a main line or tank was ruptured, the coach might get to 25 lbs at most.

Simulate it on your own coach by opening different tank drain valves and then run it up.

An open wet tank drain valve, used to do this to demonstrate the transit installed emergency air brake release circuit, the gauge on a dual brake system in a GM 5307 wouldn't make it past 30lbs.

With practice, you can set any tank drain valve to simulate a minor failure, to see if the trainees are actually paying attention, or just spewing forth memorised rote...

Nope, never made it into the curriculum, the rest of the trainers would have to know what all this means....

Training course can't be any smarter than the trainer...

Happy coaching!
Buswarrior
Frozen North, Greater Toronto Area

Offline Dave5Cs

  • Hero Member
  • *****
  • Posts: 3899
  • BCM Subscriber
Re: Mc8 won’t air up past 60 psi
« Reply #21 on: November 04, 2019, 04:37:15 PM »
Change the air Governor. They are about 10.00 and lots of times thats all it is if you don't hear or see any other leaks. Check the leveling valves for leaks too.
"Perfect Frequency"1979 MCI MC5Cs 6V-71,644MT Allison.
 1998 Jeep TJ & 2002 Buick Century, Roseville,CA
Congressmen should wear uniforms, like NASCAR drivers, so we could identify their corporate sponsors

Offline SeanM

  • Newbie
  • *
  • Posts: 33
Re: Mc8 won’t air up past 60 psi
« Reply #22 on: November 04, 2019, 04:41:43 PM »
When I bought it it didn’t run. Was a stuck rack. Was able to get them all free and turned over no problem. Then came the air problem lol. Always something it seems. Guess I better get used to it.

Agreed now that it’s home I can take my time and go through it without feeling a time crunch. The new tank will be here this week. Will replace that then check everything you all are suggesting.

Offline Jim Eh.

  • Hero Member
  • *****
  • Posts: 919
  • Excess in moderation
Re: Mc8 won’t air up past 60 psi
« Reply #23 on: November 04, 2019, 05:13:56 PM »
Reaching 60lbs and no further suggests the leak is on the aux side. If a main line or tank was ruptured, the coach might get to 25 lbs at most.

Could be but it would seem that it would be quite audible at that point. I have had mainline leaks sized so that the system will reach 60 and when the aux valves open it will build no more. This is the reason it drove us nuts trying to find the issue. But you are probably correct and in this case the problem lies in the aux side.
"Some days it's just not worth chewing through the restraints"
Jim Eh.
1996 MC12
6V92TA / HT741D
Winnipeg, MB.

Offline Bus Lurker

  • Newbie
  • *
  • Posts: 20
Re: Mc8 won’t air up past 60 psi
« Reply #24 on: November 04, 2019, 08:26:55 PM »
Sean,

Having the coach hauled away was a great solution.  Just out of curiosity what's the going rate to hire a Landoll?

Mike

Offline SeanM

  • Newbie
  • *
  • Posts: 33
Re: Mc8 won’t air up past 60 psi
« Reply #25 on: November 05, 2019, 05:22:21 AM »
It cost me 500. Was towed 35 miles. Charged by hour. Took about 4.5 hrs due to traffic. Awesome guy to work with very fair and willing to go the extra mike. Other quotes I git where 900+ and non of them where willing to to it due to it not airing up.

ALFIE'S HEAVY TOWING & TRANSPORT

6408 Causeway Blvd, Tampa, FL 33619
(813) 917-9523


Offline Ed Hackenbruch

  • Hero Member
  • *****
  • Posts: 2818
Re: Mc8 won’t air up past 60 psi
« Reply #26 on: November 05, 2019, 08:38:32 AM »
The reason i mentioned checking the ping tank is a few years ago i helped a GM owner who could not build air pressure past 30 or maybe it was 60 lbs.  We spent several hours trying different things looking for leaks. Turned out he had never drained his ping tank and it was plugged full of what looked like reddish sand which was probably really just rust.  Used a screw driver and a piece of stiff wire to get the stuff out....fired up the bus and it built air just fine.  Even though it says right on it to drain daily, or in our case, after anytime you have driven it, most people ignore it. Don't just open the drain, stick something up there and see if anything comes out.
Used to own a 1968 MCI 5A with 8V71 and Allison MT644 transmission.

Offline Boomer

  • Hero Member
  • *****
  • Posts: 1015
Re: Mc8 won’t air up past 60 psi
« Reply #27 on: November 05, 2019, 08:03:11 PM »
Nice to see a trucker who isn't afraid of an over height load.
'81 Eagle 15/45, NO MORE
'47 GM PD3751-438, NO MORE
'65 Crown Atomic, NO MORE
'48 Kenworth W-1 highway coach, NO MORE
'93 Vogue IV, NO MORE
1964 PD4106-2846
Vancouver, WA USA