| Author |
Message |
Rick Johnson (Plywud)
Registered Member
Username: Plywud
Post Number: 8
Registered: 9-2007
Posted From: 76.104.236.71
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, October 13, 2007 - 1:45 pm: | |
I have a 79 MCI 5C 6V71 644 Allison, have had it just over 7 years. Went out this am to take her for a ride, charge the batteries, just take a run and it normally takes about 10 minutes to fully charge the air system. I usually start from about 30 psi if she been sitting for very long, 30 to 60 psi happens pretty quickly, just a couple of minutes, historyicaly 60 to 90 psi takes the longest, and then 90 to pop off fairly short. This morning I did my due dilligance that I normally do while waiting, came back and it was sitting on 90 psi. Pulled out and the compressor began charging up to 120, but would bleed back down to 90 in less than a minute, charge back up and bleed down again. This usually takes 10 to 15 minutes depending on how much I use the brakes.
What happens between 90 and 120 psi's? I'm assuming that pre 90 psi's is air bags and holding tanks. Could not hear anything leaking to any great extent, nothing that would bleed down the system that quickly. I made sure it was charged up to 120 before I shut her down, nothing underneath, slight hissing in the engine compartment, but when I went back to check the pressure was still close to 120 after 4 or 5 minutes.
Any ideas?
RJ |
George M. Todd (George_mc6)
Registered Member
Username: George_mc6
Post Number: 273
Registered: 8-2006
Posted From: 207.231.75.253
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, October 13, 2007 - 2:44 pm: | |
Rick,
Most busses have a protection valve which prioritizes air to the brake reservoirs. It sends all compressor output to the brake reservoirs until they reach about 60-70 lbs. Then it opens to allow air to the auxiliaries. It then takes time to air UP the bus, literally and figuratively. The air bags hold a lot of air, which is why the long time between 60-90.
It sounds like you have developed a leak between the compressor and the first reservoir, as there are check valves in the system to keep brake reservoir air from escaping back thru broken pipes, etc. The gauges indicate brake res. pressure.
Get pressure up, get out the spray bottle and liquid detergent, and DON'T GO UNDER THE BUS, OR BETWEEN THE TIRES AND FENDERS WITHOUT PROPERLY BLOCKING IT AT THE JACKING POINTS!!!! We want you alive. Please read the post about 30 lines down, "Man killed after being pinned under bus."
George
(Message edited by George Mc6 on October 13, 2007) |
Buswarrior (Buswarrior)
Registered Member
Username: Buswarrior
Post Number: 1085
Registered: 12-2000
Posted From: 74.12.84.122
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, October 15, 2007 - 9:36 pm: | |
Hello Rick.
I'm suspicious of your parking brake circuit.
Try this: block the wheels, release the parking brakes, shut down the engine and then listen...
Only difference between sitting still shut down and going down the road is the parking brake circuit pressurized.
Let us know where the air is leaking out, and we can trouble shoot from there.
happy coaching!
buswarrior |
Rick Johnson (Plywud)
Registered Member
Username: Plywud
Post Number: 9
Registered: 9-2007
Posted From: 76.104.236.71
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, October 18, 2007 - 1:49 am: | |
Buswarrior, thank you. What made you think of the parking brake? I had left the bus out on the street so I could mow the back yard for the last time this year. When I went back out to move it, the pressure was back to normal. This had happened to me once before, about 3 years ago. I though about that as I was putt'n her away, listened when I shut down, but din't hear anything, but I had set the brake. Will try it again this week-end, and try your test. Thanks
RJ |
James Stacy (Jimstacy)
Registered Member
Username: Jimstacy
Post Number: 111
Registered: 1-2001
Posted From: 75.40.239.186
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, October 21, 2007 - 8:35 pm: | |
Rick,
Did you do the "required air checks" in the pretrip? This is no place to screw off. You and yours could be seriously injured if your brake system failed. Check the archives for an excellant pretrip inspection list and follow it. You might really be glad you did, one day.
Several times as a tour bus driver, I refused to take out a bus with a leaking brake diaphragm. The mechanics asked how I knew. Seems the other "professional" drivers never checked!
I'll get off my soap box now. Be careful out there.
Jim Stacy |
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