RoadTripper2084 Posted June 12 Posted June 12 (edited) 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 Posted June 12 Posted June 12 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 Posted June 12 Author Posted June 12 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 Posted June 12 Posted June 12 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. 4
Ivan K Posted June 12 Posted June 12 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. 2
Benjamin Posted June 12 Posted June 12 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 Posted June 12 Posted June 12 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 Posted June 12 Posted June 12 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 Posted June 12 Author Posted June 12 (edited) 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 Posted June 12 Posted June 12 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. 2
RoadTripper2084 Posted June 12 Author Posted June 12 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. 1
dl_racing427 Posted June 12 Posted June 12 Definitely do NOT remove a plug, under pressure. It can become a dangerous projectile with surprising force. 2
dandick66 Posted June 13 Posted June 13 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 1
RoadTripper2084 Posted June 13 Author Posted June 13 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. 1
96 EVO Posted June 13 Posted June 13 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' 😬!! 2
Ivan K Posted June 13 Posted June 13 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 Posted June 13 Posted June 13 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 Posted June 14 Posted June 14 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 Posted June 15 Posted June 15 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.
klcdenver Posted November 29 Posted November 29 I hope it is okay to add to this forum since this is similar problem. I am have a problem in this area but not sure what it is. I think a stuck internal check valve in the secondary tank. Performed the Motorhome Air Brake Pre-Trip Check and found the following problems: 1. The secondary tank only empties to 115-120 PSI on the gauge by opening the drain, the Red Low Pressure Warning light illuminates like it has fallen below 60 PSI even thou gauge shows about 120 PSI. Can fan the brake pedal then the secondary tank empties to 0 PSI and the Parking Brake engages at about 30 PSI. Note when fanning the brake pedal the gauge shows the drop in pressures. 2. Primary tank only pressurizing to 100 PSI parked even bringing up to 1500 RPM's. When driving it pressurizes to 125 PSI. Have checked for leaks and found a few small leaks and fixed them. Replaced the Check Pressure Protection Valve and this seemed to start all the problems. Suggestions or the correct fix appreciated.
96 EVO Posted November 29 Posted November 29 Your secondary dry tank won't have an internal valve. That's only in a split tank, as in your wet/ primary dry tank.
Frank McElroy Posted November 29 Posted November 29 The primary brakes are the drive axle brakes. The rear primary air tank feeds the drive axle brakes. The secondary air tank feeds the steer axle brakes and that is typically a combined wet/secondary tank with an internal check valve.
96 EVO Posted November 29 Posted November 29 3 minutes ago, Frank McElroy said: The primary brakes are the drive axle brakes. The rear primary air tank feeds the drive axle brakes. The secondary air tank feeds the steer axle brakes and that is typically a combined wet/secondary tank with an internal check valve. I'll have to check my manual. They may have the primary and secondary labeled wrong. Wouldn't be a shocker 😁!
Frank McElroy Posted November 29 Posted November 29 43 minutes ago, klcdenver said: I hope it is okay to add to this forum since this is similar problem. I am have a problem in this area but not sure what it is. I think a stuck internal check valve in the secondary tank. Performed the Motorhome Air Brake Pre-Trip Check and found the following problems: 1. The secondary tank only empties to 115-120 PSI on the gauge by opening the drain, the Red Low Pressure Warning light illuminates like it has fallen below 60 PSI even thou gauge shows about 120 PSI. Can fan the brake pedal then the secondary tank empties to 0 PSI and the Parking Brake engages at about 30 PSI. Note when fanning the brake pedal the gauge shows the drop in pressures. 2. Primary tank only pressurizing to 100 PSI parked even bringing up to 1500 RPM's. When driving it pressurizes to 125 PSI. Have checked for leaks and found a few small leaks and fixed them. Replaced the Check Pressure Protection Valve and this seemed to start all the problems. Suggestions or the correct fix appreciated. Air up both tanks then with the engine OFF, quickly dump the wet tank. Both the primary tanks should hold air. If they don't, the tank that drains with the wet tank has a bad check valve. Then after draining the wet tank and with both dry tanks holding air, drain one of the dry tanks. The other tank should hold air. Then air up and repeat draining the wet then the other dry tank. That should check all the tank to tank check valves. 2 minutes ago, 96 EVO said: I'll have to check my manual. They may have the primary and secondary labeled wrong. Wouldn't be a shocker 😁!
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