Air pressure drop between 120psi and 105psi

Jinxie · September 21, 2020, 06:44:04 PM

Hi Folks
First post.....I recently purchased a 77 MCI MC8 with an 8v71 and a 4 speed transmission. Somewhat of a conversion....I call it more of a day tripper. Has a generator, invertor, holding tanks, washroom, hwt, sink, etc. Just not a typical bedroom in the back setup. Perfect for hanging out at the beach on nice days!
Anyways....I will have numerous questions Im sure.

First one.....my air pressure builds up fine. As soon as it hits 120psi and the drier purges, you can watch the air gauge sink pretty fast back down to 105psi where the compressor cuts in again. It takes about 90 seconds to drop from 120psi to 105 psi.

If I shut the bus off at 120psi.....it still takes 90 seconds to drop to 105psi, but it seems to hold 105 and drop ever so slightly over the next few hours. In fact, after 2 hours it was down to 60psi and both the front and rear suspensions were still aired up.

I have to rig up a ramp system to get under the bus and check the drier. Maybe some kind of valve is hanging up?? Any suggestions on where to look?

Thanks!

thomasinnv · September 21, 2020, 07:52:42 PM

Sounds like the purge valve may be leaking.

dtcerrato · September 21, 2020, 07:57:46 PM

To my knowledge most of the 15 lb. air loss is a function of the drier. Besides the purge there is about a 10 lb, air bleed off to dry the desiccant.
Reference the NOTE on page 9 of the attachment.

dtcerrato · September 22, 2020, 04:33:22 AM

Reading over your last paragraph - don't get under the bus without solidly blocking the bus body so it can't drop with decreased air in the suspension causing death.

Jinxie · September 22, 2020, 06:00:22 AM

hogi6123 · September 22, 2020, 08:19:52 AM

There could be air leaks in a lot of different places. But at that rate it's almost guaranteed you will hear them, easier with the engine off.

My pressure doesn't drop when the air dryer purges. I thought the purge is only from the compressor to the dryer and shouldn't involve the air tanks or pressure gauge at all.

Definitely make it safe and get under there and listen for hissing air.

bandsaw · September 26, 2020, 09:23:15 AM

On my Prevost there is a pressure protection valve set at 85 psi. The air will drop to the point the valve closes and then it slows down. This valve is to keep air to the brakes if there is big leak in non critical systems.