Air problems

quantum500 · September 03, 2008, 08:21:52 PM

'74 mc-8 for a while it took a real long time to get air then everything was fine. Last time I started it I got no air, hooked it up to the shop compressor still no air. I'm pretty sure I have a check valve stuck but I tried looking before it got dark tonight and had no luck. I have a manual in pdf but I can't read that particular schematic. First if anybody knows where to find the check valves that would be great. Second if anybody had any other ideas that would be great to. Thanks!

JackConrad · September 04, 2008, 05:59:55 AM

You say no air. Where are you checking for air pressure, "at the gauge?". The gauge is connected to the accessory tank (the last tank to get air). Where are you connecting the shop air to the bus? Have you tried opening the drain valve on any of the other tanks to see if there is air pressure there? I would start with the ping tank (first tank after the air leaves the compressor). If no air here, problem is in compressor/govenor. If air pressure at ping tank, go to wet tank, etc. Hope this helps, Jack

quantum500 · September 04, 2008, 07:20:00 AM

For the moment I can't run it so I'm just looking for ideas. I was giving air to it through the valve in the engine bay. If I bled it back from that point it seemed as though I just had a line full vs a tank. I need to get my current project done so I can move onto blocks. Any other ideas are welcome, and if anyone has a diagram they could most of the air system that would be great also.

chvyman · September 04, 2008, 08:16:30 AM

Mine was doing the same thing and it turned out to be the pressure protection valve by the front tanks it was alot of joy getting to it ,took four days to jack it up high enough to crawl underneath it . The valve itself was only about $35.
Bob Chvyman

buswarrior · September 06, 2008, 09:52:44 PM

If you have an air drier, and the purge valve sticks (or freezes) in the purge/air compressor cut-out position, you will get no air past it. The fill line in the engine room is ahead of the air drier, so won't help.

Long steel rod against the bottom part of the air drier in through a wheel well and whack it with the hammer might convince it to move... you'll want to do a tear down to prevent it from re-occurring.

happy coaching!
buswarrior

quantum500 · September 07, 2008, 08:57:13 PM

I got it running this evening so I have some more info. It pumps up very slowly purging every minute or so and faster on high idle. The fill line in the engine bay is running 130psi. The parking brake will disengage, the driving brakes have good feel. The dash gauge shows low and the low air buzzer is going off, until the dash gauge hits 90psi wich takes at least 30 min. The purging continues but I seem to have good air pressure every where except the gauge and the buzzer. I don't get it. But I will assume that the dryer is to blame?

Lin · September 07, 2008, 09:14:41 PM

If your gauge and buzzer come off the accessory tank, maybe you should look for a problem there: stuck check valve, leaks in tank, leaks in lines?

letz4wheel · September 07, 2008, 09:25:47 PM

The purging cycle is triggered by the governor on the compressor from what i understand. You might want to try that first. It is probably best if you service the drier also. On the semis I run we service the dries about every 300,000 miles or so. Who knows when yours was last done. after all that you might want to look into the pressure protection valve.
Most of these things you can bypass. That would be the best thing for trouble shooting.

buswarrior · September 08, 2008, 01:22:32 PM

More forensics required before intervention.

You need to observe whether the pressures are cycling in the "good" parts of the system in any sort of relationship to this frequent purge of the air drier.

Install suitable fittings in order to use some kind of pressure gauge.

A suggestion: Regular male airline fittings installed in each air tank drain allows both a shop airline or a gauge with a female connector to be quickly and easily installed, both in the driveway, and out on the road during "pneumatically challenged" moments.

If the pressures are cycling up and down by 20 - 25 pounds, so in your case, up and down from 105 to 130, then the governor and air drier are doing what they are intended to do. You really want a reading off the wet tank.... hmmm, in a 1974 MC8.... do you know which tank? My book is temporarily MIA....

Just because the brakes function, that is no indication as to how much air pressure might be behind them.

Next question is: Is the air leaking out? Where, and how heavy?

brake valves, air lines, air suspension, radiator shutters/dampers, air wipers, whatever else you have that has been plumbed or unplumbed, now or in the past, rightly or wrongly, by previous owners or the manufacturer.

I wouldn't be worrying about carving into the system or spending money using the shotgun parts changing approach until you have thoroughly and inexpensively evaluated where and at what pressures the air is moving around inside, or escaping, the system.

happy coaching!
buswarrior

quantum500 · September 08, 2008, 02:06:18 PM

Yeah I'm going to check it out more tonight. I made some blocks so I can get under it and check out the dryer. I like your idea of valves on the tanks! I have no leaks that should be causing any problems other than the frequent purging action. I have never approached anything with the "shotgun parts changing" method. If anything I'm guilty of not buying parts if I can get it to work with out.

quantum500 · September 08, 2008, 08:20:45 PM

Another weird thing happened. To make things fast I filled the emergency fill for the parking brake, parking brake would not disengage. Hooked air to fill in engine bay gauge moved fairly quickly and stopped at just over 30psi. Started engine to make air and nothing was happening according to the gauge. Checked it a couple minutes later and I had 75psi. Watched my gauge for a while and nothing was happening so I started hitting the brake peddle over and over again it ran the dash gauge down to 25psi before it stabilized. After that it would not make pressure at all, I still had shop air hooked up. So I blocked the bus up and crawled under it and the purge valve is not going off its the pressure protection valve and it is blowing oil and water all over. So...can the purge be blocked making a reservoir of oil and water blocking some of the air ports? A couple hours later I crawled under and pulled the pressure protection valve and it ran a good stream of oil water out.

hargreaves · September 08, 2008, 08:49:30 PM

Chances are you have a plugged ( usually with carbon) discharge line or ping tank. disconnect the air discharge line at the ping tank and start the engine. If their is lots of air coming out go to the next connection until you find the blockage.

Gerry

buswarrior · September 09, 2008, 01:34:48 PM

ok, we have trouble...

Take some pictures or call the components by their proper names or this is just a jumble of BS.

The "purge valve" - is this at the bottom of the air drier or somewhere else?

The real "pressure protection valve" does not have facility to drain anything. It is sealed, with an adjustable cap that screws up or down to change the pressure at which it allows air deeper into the air system.

By "discharge line" which line do you mean?

What do you mean by "ping tank" and where is it?

Trouble shooting from a distance is impossible without common terms.

happy coaching!
buswarrior

hargreaves · September 09, 2008, 07:44:02 PM

The pressure protection valve allows the air pressure to escape if the pressure gets to high ( over 150psi.) usually because the unloaders on the compressor are stuck. this protects the air system. it is usually located near the compressor discharge line in front of or after the ping tank.

The pig tank (if it has one) is placed directly after the compressor discharge line. the ping tank is used to soften the compressor pulses and cool the compressed air coming from the compressor. this reduces the condensation that forms when the air cools.

quantum500 · September 09, 2008, 07:59:08 PM

Quote from: buswarrior on September 09, 2008, 01:34:48 PM
ok, we have trouble... this is just a jumble of BS.
Trouble shooting from a distance is impossible without common terms.

happy coaching!
buswarrior

I will agree and we could have a jumble of BS. I had to look up ping tank and every definition of the term I could find is a tank to cushion the air ride. Usually one tank per corner. So on to my problem and I hope the right terms. From what I can tell the purge valve has not been working on the dryer, about mid way up on the dryer is a fitting that I'm assuming is the pressure protection valve or at least a pressure protection valve for the dryer. Bear with me I'm no air brake specialist. I have been reading the bendix hand book though so I'm getting better

. So the valve in question that I'm calling the pressure protection valve has been going off on a regular basis blowing water and oil all over. I'm assuming a accumulation of crap that the purge valve has not been getting rid of. Yesterday I pulled the pressure protection valve at low pressure and got a good steady stream of goo to come out after I did that I'm getting better pressure, although I did not have time to get into the forensics of it what ever I did helped the problem. So my question is if the purge valve has not been working correctly can the accumulation of goo in the dryer cause my problems?