Primary & Secondary Air Circuits - In theory and in practice

[RoadTripper2084]

Okay folks, I was under the coach lubing things up and contemplating replacing my air dryer filter so as a test I performed the following (Spring/parking brakes were applied during all steps below):

Front/Primary Tank

1. Aired up the coach, both primary and secondary (front and rear in this case) tanks were steady at 125 PSI after the dryer purged. Engine was shut off.

2. Opened the drain petcock on the dry-side of the front tank (primary tank) fully and waited for all air to expel out (note expelled air was very dry, maybe two drops of condensation on my hand).

3. Checked the air pressure gauge on the dash, which showed the front/primary tank was now completely empty, and the rear/secondary tank was still sitting nicely at 125 PSI.

Observation: This seems normal/expected as the wet/supply tank would have tried to fill the quickly emptying primary tank as it drained, eventually draining itself in the process(since the engine wasn't running and hence, no air compressor to supplement the wet tank's air supply).

Wet/Supply Tank

1. Aired up the coach, both primary and secondary (front and rear in this case) tanks were steady at 125 PSI after the dryer purged.  Engine was shut off.

2. Opened the drain petcock on the wet-side of the front tank (Supply tank) fully and waited for all air to expel out (note expelled air was completely dry).

3. Checked the air pressure gauge on the dash, which showed the front/primary tank was now completely empty, and the rear/secondary tank was still sitting nicely at 125 PSI.

Observation: I would have expected that draining the wet/supply tank only should NOT have also caused the Front/primary tank to drain. Shouldn't there be a one way check valve or pressure protection valve between the wet/supply tank and the front/primary tank to prevent a leak in the supply circuit from also draining the primary circuit? Confusingly, looking at pg. 5 of the attached air system diagram pdf, I don't even see the connection between the wet/supply side and the dry primary side of the tank shown?

Rear/Secondary Tank

1. Aired up the coach, both primary and secondary (front and rear in this case) tanks were steady at 125 PSI after the dryer purged.  Engine was shut off.

2. Opened the drain petcock on the rear tank (secondary tank) fully and waited for all air to expel out (note expelled air was completely dry).

3. Checked the air pressure gauge on the dash, which showed that BOTH the front/primary tank and rear/secondary tanks were now completely empty.

Observation: I would have expected that draining the rear secondary tank only should NOT have also caused the front primary tank to drain. There is supposed to be a "5/8" RED FROM ONE WAY CHECK (REAR) TO WET TANK" as per the diagram on pg. 3 of the attached PDF.

So, logically, it would seem that a failing one way check valve between the primary side of the front tank and the wet/supply side would cause the failure pattern that I am observing:  open rear tank drain causes supply tank to drain into rear tank (correctly), but failed check valve also allows the primary tank to backfill into the wet/supply tank, resulting in it also draining (incorrectly).

Just wondering if anyone can confirm this (or provide additional insights)?

I haven't looked at my physical front tank connections yet to see if I can figure out how the wet/dry sides actually connect, but I assume it should be pretty obvious once I'm staring at it.

roadmasterAir20090611135315489.pdf

Edited June 12 by RoadTripper2084

[Frank McElroy]

First, your primary tank is the rear air tank.  The secondary air tank is up front with a failed internal check valve between the dual wet and secondary front air tank.  To remove and replace this check valve will require tank removal to gain straight access to this internal check valve through the wet tank side end.

[RoadTripper2084]

Thanks, you are absolutely correct re: primary vs secondary, I was just reviewing some air brake documentation and saw that I had them confused.

So the connection between the wet and dry side of the tank is *inside the tank*?  How exactly does one access the internal check valve (with the tank removed)?

 

[Frank McElroy]

Let me look it up.  I have a diagram.  You remove the tank.  Remove the plug on the wet side.  A long extension to gain access to the internal check valve and then unscrew it.

But, have you first tried airing up and quickly just dump the wet tank a few times to possibly unstick the internal check valve?

BTW, hats off to you for checking these air tank check valve safety devices to make sure that your primary and secondary braking systems function as designed.  Most folks don't realize the importance of checking that their primary and secondary braking systems actually work independently as designed should either system fail.

[Frank McElroy]

This document should explain what you need to do.

 

[Ivan K]

I don't know if this would work for you but on ours I have access to the side plug and a straight shot to check valve between the frame and h-frame with the wheel removed and the bags supported just right. Hopefully you get the valve working with a bigger blast than the little drain valve can do.

[Benjamin]

If you have an "external check valve", this is what I expect it to look like: https://www.monacoers.org/topic/8604-air-leak-and-unknown-bolt/#comment-90665
It's not external at all, just mounted so it's more easily accessible.

 

[Bill R]

3 hours ago, RoadTripper2084 said:

3. Checked the air pressure gauge on the dash, which showed that BOTH the front/primary tank and rear/secondary tanks were now completely empty.

For my coach's air system, there is a shuttle valve that will isolate the Primary and Secondary tanks when one or the other loses air.  Here is a link to the shuttle valve and it is mounted on the secondary tank.   Not sure if your system is the same.  If it is, then I would suspect you may have a faulty shuttle valve causing the secondary tank to lose air when the primary tank is drained.   https://www.ebay.com/itm/321151003190

[Ivan K]

I have the same valve, if the leak isn't large enough, the plunger will not necessarily close. It could be that opening the drain valve just wasn't enough.

 

[RoadTripper2084]

12 hours ago, Frank McElroy said:

This document should explain what you need to do.

 

Thanks for this! 

Quote

Take care not to drop the check valve into the reservoir when removing or installing it. The res. ervoir can not be used if the check valve is lost in it. A loose check valve could scratch the tefion coating of the interior of the reservoir, causing it to corrode. If the check valve is not recoverable, replace the reservoir.

Ugh.  

Somebody was having a bad day at the tank design office when they decided an internal check valve was a good idea.

It does say that some units use an external connection. Hopefully mine is one of those, as I give my chance of success of getting the old valve out, without dropping it in the tank, and another in, without dropping it in the tank, at about 20% on a good day.  I'm in software, UNDO is my speciality. 😉

 

11 hours ago, Benjamin said:

If you have an "external check valve", this is what I expect it to look like: https://www.monacoers.org/topic/8604-air-leak-and-unknown-bolt/#comment-90665
It's not external at all, just mounted so it's more easily accessible.

 

...I just checked some photos I have of mine, I don't appear to have that large bolt in top of the center of the tank as you have, unfortunately.

13 hours ago, Frank McElroy said:

But, have you first tried airing up and quickly just dump the wet tank a few times to possibly unstick the internal check valve?

This is a good idea. Maybe if I remove end plug from the wet end of the tank I can use my air compressor to blast the valve with higher pressure air to try clear any debris. Also, removing the side plug when it is pressurized would trigger a much larger pressure drop over a short period of time, which might loosen it up.

In the document you posted they mentioned that it can fail closed (so no or low air from wet to secondary side), in which case it is acceptable to install a bypass. I assume that means using available ports to add a line between the tanks with the valve connected between them (externally). Not that this would solve my problem anyway.

Edited June 12 by RoadTripper2084

[Benjamin]

The problem with trying to clear the valve is that it's in the middle between two tanks, that both will change pressure slowly.  And you need volume, not pressure to clear it, the pressure difference makes the volume.  You don't want to remove a plug under pressure, but you could add a ball valve, charge it up, and open the ball valve to get good flow. 

[RoadTripper2084]

23 minutes ago, Benjamin said:

The problem with trying to clear the valve is that it's in the middle between two tanks, that both will change pressure slowly.  And you need volume, not pressure to clear it, the pressure difference makes the volume.  You don't want to remove a plug under pressure, but you could add a ball valve, charge it up, and open the ball valve to get good flow. 

Thanks for the suggestion, I like the ball valve idea.

[dl_racing427]

Definitely do NOT remove a plug, under pressure.  It can become a dangerous projectile with surprising force.

[dandick66]

When I had air issues last year I thought my internal check valve was bad.  Turns out it was something else, but I found this document.  Look at the last page, it shows how to bypass the internal valve.  I agree that your chances of removing that valve without dropping it are high.  

freightliner split tank.pdf

[RoadTripper2084]

Thanks.  The bypass is for when the check valve sticks closed, preventing air from moving from the wet side to the dry side of the tank.  In my case, the dry side fills fine, but it appears that the check valve doesn't close when the wet side loses pressure.  So in this case, the bypass technique would not solve my problem, unfortunately.

 

[96 EVO]

Yep, that looks like a high risk procedure replacing that internal valve!

Maybe practice for a few days on the old children's game 'Operation' 😬!!

[Ivan K]

Piece of sticky tape in a tight socket and the socket taped to extension will do it safely if it really comes to it. But still hopeful that a blast of air would get the valve working. The drain valve is too slow as the pressures keep equalizing.

[Frank McElroy]

Since in most cases to gain access the tank would need to be removed, tape the socket to the extension and position the tank vertically so in effect you are coming in from the bottom.  That way the check valve is less likely to fall out of the socket.

I still think you should air up the system, shut off the engine, and try dumping just the wet tank.  Worth a shot to see if you can get that internal check valve to work.

[96 EVO]

1 hour ago, Frank McElroy said:

Since in most cases to gain access the tank would need to be removed, tape the socket to the extension and position the tank vertically so in effect you are coming in from the bottom.  That way the check valve is less likely to fall out of the socket.

I still think you should air up the system, shut off the engine, and try dumping just the wet tank.  Worth a shot to see if you can get that internal check valve to work.

I was thinking about that today!

Lot's of work removing the tank completely, but would be the absolute best chance of the surgery going well!

[dandick66]

On 6/13/2024 at 10:59 AM, RoadTripper2084 said:

Thanks.  The bypass is for when the check valve sticks closed, preventing air from moving from the wet side to the dry side of the tank.  In my case, the dry side fills fine, but it appears that the check valve doesn't close when the wet side loses pressure.  So in this case, the bypass technique would not solve my problem, unfortunately.

 

Actually, you are only bypassing the internal check valve.  If you look at the drawing, you will be installing a new, external check valve.  The check valve is supposed to allow air to move from the wet side to the dry side.