Air leak solved

[Chopper Scott]

for the moment anyways. I have always lost air pressure when shut down. Generally after 20 minutes or so I start to hear the wiper controls start leaking once the dash gauge gets to 30 pounds. I have never found any obvious leaks other than the occasional height controls leaking down to relevel the bus. The bags stay up and such and i have no problems with the compressor keeping up while running or even airing up the system upon start up. This weekend it was some 4 hours after shutdown that I heard the wiper controls leaking and I looked and it was just getting down to the 30 lbs that triggers them to leak off. Today after shutdown and unloading the bus I looked at the dash gauge and it's sitting at 100lbs after several hours. Any ideas? I know prayers work but I haven't been praying about this!!

[jbnewman]

My first thought, upon reading your post, was, "isn't 30 psi about the point at which the parking brake would kick in?"  I'm guessing that if this is an air brake system, that you're doing full pre-trip brake checks, that would catch problems in that regard, but, just in case,

• When the bus is running, and air compressor not running (empty the system by applying the brakes several times, wait for it to re-fill, then cut out, at which point you know it's not running), is there any air loss?
• When you push the brake down several times in succession, does the low pressure warning come on at about 75 psi?
• Finally, at about 30-40 psi, does the parking brake release?

-jbn

[Chopper Scott]

No and no and no I guess. I really have no issues other than after shutdown the air dropped down and the wiper controls would start to leak signaling the pressure had dropped to 30 lbs (for some reason). The park brakes don't release until I air it up and release them. The bus airs up quickly and I have no problems with a lack of air pressure when running. In fact I just assumed it was a normal condition except for reading from others that their pressures stayed up for days and days.... Now it seems to have cured itself. Strange.

[RJ]

JBN & Scott -

OK, a little confusion going on here.  Let me see if I can thoroughly fog up the brains. . .

Scott - JBN was talking about the standard CDL pre-trip inspection air brake check, a good copy of which is here, near the end:

http://www.busnut.com/bbs/messages/12262/16203.html?1167072614

JBN - I think you confused Scott with your terminology.  The parking brake doesn't release at 30 - 40 psi, it should automatically set or apply at approximately that pressure.  On an MC-7, it takes a full service brake application with over 100 psi showing on the dash gauge, held for 3 -5 seconds, to release the parking brake after pushing in the knob. (DD3 brakes)

Scott -  Unless I'm totally "out to lunch" (and some folk wonder if I am), I think that maybe the reason the system seems to have cured itself since your original post may be due to the weather!  (Huh?) 

Did the system leak down faster during the winter months?  Now that it's warmer, it appears that the system is leaking down more slowly?

As a guy, you know that things swell and seal better when they're hot, and that's probably what's going on with your 7's air system.  Give it a cold shower, everything shrinks, thus increasing the potential for leaking.

But you've got a slow leak somewhere, however, I wouldn't be alarmed.  If the coach passes a CDL air brake check ok, then the leakage is more of a nuisance than a hazard.  I've seen brand new transit buses, just delivered from the factory, that lose their air pressure within a couple of hours, yet pass the air brake check with flying colors.  Part of the wonderful world of air-suspended buses.

If it really bugs you, then it's time to get out the spray bottle with soapy water and start squirting.

FWIW & HTH. . .

[luvrbus]

Scott check the shut down cylinder on the engine those things play tricks all the time lol


good luck

[robertglines1]

could be something as simple as passing of a piece of trash out of a ck valve. A ghost of a problem is always hardest to find and fix. Enjoy it while it last! I once isolated my wiper motors with a shut off air valve to solve leak down problems over time. needed to turn valve on before using wipers.  Never could hear them leaking.   Bob

[Van]

I shrink and leak after a cold shower all the time! nothing to it Beer... go figure!

[Don4107]

Could it be that you are using the bus more lately and it likes it?  Lots of things work better on these things when they get regular exercise. 

Keep it up,
Don 4107

[Chopper Scott]

I think I'm going to take Bob's advise and enjoy it while it lasts. Thanks for all the theories however. This is my 3rd summer of owning it and it has always leaked down, hot or cold, used a lot or used a little.Glad to hear it's nothing to worry about.

[gus]

RJ is right, my 4104 suspension stays up four times as long in hot weather as in cold.