GMC Wet Tanks

gus · June 19, 2006, 01:43:11 PM

Why are GMC air system wet tanks all the way at the rear of the bus? Maybe they aren't all like that but my 4104 is and I would guess that all the others of that era are also.

Trucks have both tanks close together, sometims just a few inches?

I thought it was back there for the rear brakes but it isn't, both axles get brake air from the front tank.

Also, why is it mounted vertically instead of horizontally like all the others?

gus · June 19, 2006, 01:55:26 PM

I forgot one question, do MCIs have this setup, especially older ones like the MC5?

NCbob · June 19, 2006, 03:20:53 PM

The 'wet' tank is the first of the two in the rear of my MC5A. The moist outside air from the compressor enter that tank first and the heavy condensed droplets collect at the bottom of the tank. I have a 'quick'drain' on both tanks with a cable secured inside the wheel well out of trouble. That was it's easy to give them a tug at the end of the day and clear out the excess moisture. They're available at NAPA for about $12.

I'm on the process of adding an AD-2 dryer between the compressor and the 'wet' tank. With it's auto "popoff' valve I should see much less, if any moisture in the 'wet tank'.

NCbob

FloridaCliff · June 19, 2006, 04:38:52 PM

Gusc,

The other question is why didn't they make a valve in an easily accesible place like the other tanks?

Instead of having to crawl underneath.

Cliff

NCbob · June 19, 2006, 04:55:44 PM

I'd love to chide you on that question Cliff, like why ...doesn't a frog have wings....or...Oh, don't get me started!

Somehow I don't theing the bus Engineers had us..the end used in mind when they put these nightmares they call buses together.
But, in the end result, they did build one heckuva machine that we enjoy driving many years after their life expectancy is over.

Do you know why they always refer to a ship as..."She"? Because it takes soooo much money to keep them in paint and good running order.

Now, if you can find a parallel in there...you're a better man than I am "Charlie Brown"! ;

Gotcha Gotcha Gotcha
Bob

gus · June 19, 2006, 08:28:01 PM

Well guys, before this string gets too far off topic, remember my original question?

Why is the wet tank in the rear and the dry tank in front??

It is interesting that they are together on the MC5A which is the way trucks are configured.

I understand that the wet tank comes first, by definition, but why isn't it in the front close to the dry tank?

The wet tank drain on my 4104 isn't all that hard to reach, I don't even have to get on my knees to do it. My guess is that valve was designed to by used by mechanics while in the pit under the bus. I doubt that bus drivers drained the tanks.

Which brings up another point, the front one is impossible to reach and it is certainly dangerous to even try.

JackConrad · June 20, 2006, 04:33:57 AM

This is just a guess, but I think the wet tank is near the compressor to prevent moisture from building up in the line going to the front of the bus. Of course, this brings up the question "why is the dry tank installed in the front of the bus? Closer to the brake valve?? Jack

gus · June 23, 2006, 07:49:58 PM

Jack,

I agree with you about the front tank being near the brake valve.

However, the large, long line from the wet tank to the front is a perfect place for moisture to condense. It would make far more sense to me to have both tanks in front close together.

I was hoping some GM oldtimer would know the answer to this. It may have been just a space problem.

I get a lot of water out of my aux tank, which, theoretically, is not supposed to happen. But, my wet tank has been bypassed so that may be the reason.

rv_safetyman · June 23, 2006, 08:41:24 PM

I am not familiar with the older GMCs, but today's buses all have three tanks minimum. The first tank, as we have discussed, is the wet tank. It is usually as close to the compressor as possible. The air entering this tank is hot and there is no chance that it will freeze if it is plumbed with the line always have some slope to the tank. The other two tanks are the secondary (front) brake tank and the primary (rear) brake tanks. These tanks should be located as close to the axle they serve. The rear brake tank is located in the rear of the bus and delivers air to the service brake chambers via a relay valve. This relay valve quickly supplies the air based on a small signal hose from the brake valve.

So, you should have a front brake tank in the front of the bus and a rear brake tank somewhere close to the rear axle.

Chances are you have a fourth tank that supplies all of the accessories like wipers, air springs, etc.

Take the time to find all of the tanks and drain them. They will have both water and oil in them. Some oil is normal. Once you have drained all of the tanks (after the system is aired up), it is good practice to drain them at least every six months to make sure you are not getting excess oil or water

gus · June 23, 2006, 08:56:52 PM

rv,

My '54 GMC 4104 ("53-'59)is not like this and I don't thnk the 4106('60-??) is either.

Mine has one wet tank in the rear, a dry tank for the brakes at the front and an Auxiliary tank, also at the front, for the suspensio, horn and wipers.

Air for the rear brakes comes from the front tank. There is a rear brake relay but it makes brake application and release faster. The rear brakes do not get air from the rear tank like truck trailers.

Newer GMCs may have the three tank setup you describe but I think at least the 4106 is like mine and probably much later models too.

DrivingMissLazy · June 24, 2006, 05:54:38 AM

It would seem to me that you would want the water to condense in the tank, not in the line!
Richard

Quote from: gusc on June 23, 2006, 07:49:58 PM
Jack,

I agree with you about the front tank being near the brake valve.

However, the large, long line from the wet tank to the front is a perfect place for moisture to condense. It would make far more sense to me to have both tanks in front close together.

I was hoping some GM oldtimer would know the answer to this. It may have been just a space problem.

I get a lot of water out of my aux tank, which, theoretically, is not supposed to happen. But, my wet tank has been bypassed so that may be the reason.

gus · June 24, 2006, 12:56:43 PM

Yes, that is my point. It is better to have water condense in the long line to the front then go into a wet tank in the front close to the dry tank.

The 4104 wet and dry tanks are separated by a 3/4 inch 20' copper line so after water condenses in the wet tank it condenses more in the long line to the front dry tank. This may not happen in warm weather but it is sure to in cold weather.

DrivingMissLazy · June 24, 2006, 01:15:08 PM

I'm sorry I did not make myself clear. I think you want the moist air to go in the wet tank and condense there in the wet tank and it is then stored there. (as dry air)

The air leaving the wet tank would then be dry. If it condensed in the long line from the wet tank to the dry tank, then the moisture would go to the dry tank.
Richard

Quote from: gusc on June 24, 2006, 12:56:43 PM
Yes, that is my point. It is better to have water condense in the long line to the front then go into a wet tank in the front close to the dry tank.

The 4104 wet and dry tanks are separated by a 3/4 inch 20' copper line so after water condenses in the wet tank it condenses more in the long line to the front dry tank. This may not happen in warm weather but it is sure to in cold weather.

gus · June 24, 2006, 09:14:27 PM

Richard,

I don't think the air in the wet tank is dry. I'm no scientist but I think it is probably not possible to have dry air in the same closed container as water.

The air contains water vapor and the water in the bottom of the tank has condensed out of the water vapor.

From what I remember of HS Physics I think that if this air hits anything cold or expands then water will be squeezed out of it. The dry tank and aux tank both have drains just in case this happens.

The air going to the dry tank has some water vapor but will eventually be exhausted from the brake chambers if it doesn't condense somewhere first.

DrivingMissLazy · June 25, 2006, 06:11:12 AM

You are absolutely correct. It will not be totally dry air exiting the tank, but it will be significantly dryer than the
air entering the tank, since it does stay there for awhile and a majority of the water will have condensed out of it unless you are using a tremendous amount of air. I suspect that that may be the reason that some wet tanks are mounted vertical to reduce contact between the accumulated water and the air in the tank. There will be some moisture in the dry tank, but hopefully a very small amount.
Just my assumptions, by the way. Do not take it for gospel.
Richard

Quote from: gusc on June 24, 2006, 09:14:27 PM
Richard,

I don't think the air in the wet tank is dry. I'm no scientist but I think it is probably not possible to have dry air in the same closed container as water.

The air contains water vapor and the water in the bottom of the tank has condensed out of the water vapor.

From what I remember of HS Physics I think that if this air hits anything cold or expands then water will be squeezed out of it. The dry tank and aux tank both have drains just in case this happens.

The air going to the dry tank has some water vapor but will eventually be exhausted from the brake chambers if it doesn't condense somewhere first.