O/T, air brake question

[jaybee]

This is off-topic, but I know there are at least a couple of people here that know air brakes.  I know a little about them, enough to know what a brake chamber, slack adjuster, and S cam are, but no mechanic.  Last night I was on a charter bus at highway speed in below-zero weather.  The bus driver noticed the air brake warning light on, pulled off onto the shoulder and woke up his road repair guy.  The bus had 120psi air pressure and stopped fine, no wheel locked like a blown pancake or anything like that.  After a short conversation he pulled back onto the road at slow speed and stopped again without problem.  Trying this a couple of times the light started to fade (I could see it from my seat).  A decision was made to limp the remaining 20mi into town so we wouldn't be stranded in the country.  The driver did a nice job, drove very conservatively, left himself lots of stopping distance, and took us through town instead of around the outside on the highway so we'd have a better chance of finding a warm refuge if needed.  He got us where we were headed without further incident.  Watching the light I noticed it would remain off much of the time.  Sometimes it would flicker, then would burn brightly after releasing the brakes.  A couple of times it flashed.  It seemed to be better when he used the brakes frequently, worse when he was mostly off them as on the highway.  This is way off-topic for RRT, but my curiosity is running really high.  Any suggestions?

[enjenjo]

Ice in the system. There are several tanks that store air for different purposes. The one that the light was monitoring was not getting enough air to keep the light off. It may have been other things, but that is the most likely answer.

[Fat Cat]

I concur with enjenjo, it is also possible that it was ice in one of the ABS or modulator valves too.

[jaybee]

Thanks.  Those were the things I was thinking of.  Maybe we had brakes because some of them were getting air but not others?  The flashing light made me think of ABS since by law that's a self-diagnosing system.  Driver said usually when that light goes on it's low air pressure, which is why it was so confusing that the brakes continued to work properly.  He had a 100mi drive home after he dropped us off, hopefully they put it in a shop and him in a motel after leaving us.  Hate to think of anyone caught out in such bitter cold.  In a total air loss situation the spring brake will override and lock the drives, right?

[Leon]

I was driving a fire truck and one of the lines broke, and I lost all pedal.  The system still had air so I was travelling (quite a bit faster than one would usually drive with that much water behind him).  Luckily I had lights and siren going so I didn't hit anything till I got to a construction area where I was able to put it into some loose sand on the side of the road.    We had to drive it back to the station with the captain hanging in the engine compartment holding both ends of the broken line together using parts from a ball point pen.  I thought it would give me brakes when it broke, but the loss was so slow that the compressor kept up.

[enjenjo]

In a total air loss situation the spring brake will override and lock the drives, right?

yes, in theory anyway.

[moparrodder]

In a total air loss situation the spring brake will override and lock the drives, right?

yes, in theory anyway.

Yup , it will. Have it happen to me a couple of years ago in the freight shaker I drove but if you don't keep the brakes adjusted up they will be soft and not lock up completely.  The older Semis that I started out in didn't have park brakes that the trucks have now and only had one air valve that you pulled to shut the air off going to the trailer and that would set the brakes on the trailer so you could unhook the tractor from the trailer.  If you lost the air you had to act fast and down shift quick and get it to the side of the road and stoped before all the air was gone or you just free wheeled it till it finally stoped, would get a mite scarry at times.

 Bill