Can bad air tank drains cause low air pressure?

belfert · September 22, 2009, 04:10:39 PM

Can a leaky air tank drain valve cause my air pressure to be low both front and rear?

I've been having a minor problem with my air pressure being down maybe 5 or 10 lbs with one side a bit lower than the other. I replaced both of my front air tank drains with cable pull drains and suddenly both my front and rear pressure read exactly 120 lbs. How does a bad drain cause problems front and rear? I was not aware of either drain leaking and there was almost zero oil or moisture in the tanks.

My air governor likes to freeze up in the winter and I was planning to replace it. (Have the part) I figured the governor was causing the low air.

I like it when one change fixes a totally seperate problem.

WEC4104 · September 22, 2009, 04:56:53 PM

I would think that a leaky valve would only cause the compressor to cycle more frequently, so what you would find is fluctuations in pressure. With leaky valves replaced, it should hold 120 psi longer, but I'm not really sure why you would see a max pressure any higher than it was previously.

gus · September 22, 2009, 07:48:42 PM

What WEC said, there is no way drains could affect pressure unless they leaked faster than the comp could pump!!

By sides I assume you are talking about the air bags, they would not show differences because of slightly low air pressure overall.

buswarrior · September 22, 2009, 07:51:32 PM

WAG...

which drains did you fix? two front ones? was one the wet tank and the other the front service tank?

the speed at which air travels through the system is not instantaneous, it can be quite restricted by the check valves and their condition. A leaking valve one place can inhibit terminal pressure from being reached, when the governor is sensing the pressure in another tank that also has a leaking drain.

All bets are off when the air system is not functioning as intended.

Don't delay changing that governor, if it didn't function in the cold last year, it won't be any better this year!

SNOW is coming!

happy coaching!
buswarrior

zubzub · September 22, 2009, 07:51:45 PM

one of the things that bugs me the most when I'm fixin' stuff is the miracle cure that I don't understand. so I always make up a story as to why a system etc...suddenly works again. In this case I am going to say the comp governor was a little dirty, as the system was holding pressure, it never go to blow air in a no load situation....then you pulled the drains, and a little bit of crud got spit out of the governor valve whatever. OR your bus knew the drains weren't working and lowered the pressure a little so you would do some maintenance on the air system.

belfert · September 23, 2009, 12:39:06 AM

One of the front tanks is the secondary service tank and the other is the accessory tank. The wet tank along with the primary service tank is in the rear and those have some sort of automatic drain on them.

I am still going to replace the governor.

bevans6 · September 23, 2009, 05:00:18 AM

if you have a modern dual air brake system with a primary (rear) circuit and a secondary (front) circuit, those tanks are fed separately from the wet tank through check valves. As soon as the compressor governor reaches cut out pressure, the pressures should be equal in all of the tanks, in theory. But any use of air, including leakage, could cause the pressures to vary from one another. So if your secondary tank got a new drain valve, that would fix a problem that would cause it to show lower pressure. Same with the accessory tank drain valve particularly if the accessory tank was fed from the secondary tank - the loss would be doubled. Buswarrior's point about the speed of air propogation through the system is a really good point - the compressor cuts out when the wet tank reaches a certain pressure, fed back by a signal line. The compressor can't know what the pressure in the service tanks is. If your service tanks were leaking constantly, you'd very much expect that they would not have reached full pressure at cut-out, and by fixing the leaks you'd get back that last few pounds on your gauges (which read usually from the service tanks themselves, not from the wet tank where the compressor reads pressure from.

In other words, I would expect exactly what you saw happen to happen. Not that I would have figured it out in a hurry before hand, diagnosing things after they are fixed is a lot easier than the other way round...

cheers, Brian

belfert · September 23, 2009, 05:08:12 AM

I find it somewhat interesting that both the rear and front tanks were showing low before, and not by the same amount. The good thing is both front and rear now show exactly 120 PSI.

Jeremy · September 23, 2009, 05:42:25 AM

Quote from: zubzub on September 22, 2009, 07:51:45 PM
one of the things that bugs me the most when I'm fixin' stuff is the miracle cure that I don't understand.

I'm exactly like that as well

As it happens I was reading a story earlier this week about the development of the Pilkington method of making glass, which is now used under licence by every glass manufacturer around the world. The Pilkington brothers spent £7m in the 1950s (when £7m was an awful lot of money), and for months had huge trouble with a particular problem which suddenly disappeared overnight. They weren't sure why, but were nonetheless delighted, so immediately started producing copies of the prototype machinery in order to start large scale production of the glass. But when the 'new' machines were run the original problem was back, and only after a lot more effort did they discover a component deep within the prototype machine which had broken and 'solved' the original problem.

Jeremy