Air Pressure Alarm Question

This is where I'm at so far: Replaced both the MV3 dash valve module and the tractor protection valve, and the problem still exists. If anything, the new MV3 valve made the trailer valve pop at a higher psi; 100 instead of 80...so I went back to my old one. The yellow valve is operating properly as far as I can tell. It doesn't pop until 20psi'ish. I did buy a rebuild kit which I'll install once I'm able to solve this problem. I also find it odd that I can't find ANY literature regarding this problem. Every mechanic I've spoken to hasn't encountered this or heard of premature activation of the valve. Usually it's the other way around where the valve fails to activate at the minimum pressure.

Double Yellow - no significant air leaks as far as I can tell. My system holds pressure pretty well.

Baha - I just went ahead and bought the actual replacement model. There's no way to transfer the fittings (they seem infused into the unit) and I wasn't going to save much after buying quick connects for a screw-in type mv3 model.

Pablo - The buzzard is a lessor issue that I'm not concerned about atm, but thanks for your input. With it I know where to look should I experience a malfunction that I'm uncomfortable with.

Bendix Tech - #1) Yesterday I pretty much figured out that it has to be something upstream between the MV3 and supply/primary reservoir that's tripping at 80psi (or 100psi if using the new MV3), so your comment is reassuring in that regard. Also the trip psi is the same even when using the manual drain valve. I spent all yesterday evening just familiarizing myself with pretty much all components of the air system using the Bendix Air Brake Handbook I found online as well as various diagrams of typical systems. I'm wondering if there's a check valve between the primary tank and MV3 - there should be a direct connection between the two via T coupler right? One diagram had the line going to the brake valve first. The technical bulletin you provided DOES mention the issue I'm experiencing and provides alternate means of testing the trip pressure. But it doesn't describe why or what's making it trip at the higher pressure. And me being the expert I'd like to be after all this, would like to know...because knowing is half the battle - "Go Joe!!!" lol

#2) Regarding my governor I know that the material states that it's non-adjustable...technically. At least it's not supposed to be adjustable. And even though I've ruled out everything upstream of (before) the primary (supply) and secondary (service) reservoirs, the new governor's cut-out setting is ~140psi - higher than I'd like...and did I mention it's new? lol

Anyway, the plan for today is to try the test method suggested by Bendix Tech. I'll also try manual draining each reservoir SLOWLY and individually. I read somewhere that draining too fast might be the cause...I don't see how but it doesn't hurt to try it. If that fails I'll simply run a new airline directly from primary to MV3 bypassing anything that's causing my issue, and work from there.

 

Well I'll be a monkey's butt on crack - that alternate test method worked! And I'm not 100% about this but there seemed to be a check valve'ish mechanism going on, and (I don't entirely know how/why) I think it has something to do with the AD-IS air dryer. While looking at manuals yesterday I came across the brochure for the AD-IS and it states "integral pressure protection valves to protect each brake circuit. No reservoir check valves required." and "design innovations allow the system to maintain one brake circuit, to about 100psi...when one reservoir drain valve is opened, initially both reservoir gauges will fall; however, the pressure protection valves will close at pressures at or above 70psi, protecting the remaining brake circuit from further loss of pressure."

Further testing indicated that when I manually drained either reservoir [via drain valve] individually nothing tripped the MV3 with both yellow/red valves pressed in. This behavior indicates/verifies that there is indeed 'something' interacting between the tank and the MV3 It's only when I drain both tanks TOGETHER that my test worked identical to the one recommended by the technical bulletin; which states to block off the red gladhand, leave the blue gladhand untethered/free, push in the trailer valve, and depress the foot brake. Air will escape through the blue gladhand until both primary and secondary tanks deplete to the trip pressure. Once reached BOTH dash valves will trip/pop together. <--If you're looking for the the red one to pop before the yellow using this method, then you won't see it. Doing it the way that we're taught will result in the red valve tripping at "pressures 70psi and above". Considering that the dryer's protection valve is calibrated to about 105psi the red valve will trip around that pressure, well before any alarm sounds.

So basically nothing was wrong with my air system lol. In the end I replaced my tractor protection valve (it was cosmetically severely damaged but still functional) located at the rear of the vehicle. I did NOT replace my MV3 with a new one but since I had my dash opened and the MV3 valve out I added some quick connects and additional airline segments to give me more play should I need to remove it again in the future. Initially there isn't a lot of room/airline to work with so it can be a chore to unplug the darn thing, and if you sacrifice tubing in the process then you'll need to use unions anyway. Used some 3/8th OD tubing and quick connects. Although I have brass ones pictured here I ended up using black nylon ones from Autozone (800-193). 2/$8. You can also get some from Home Depot (and they will work) in the plumbing section. The working pressure is listed as 150psi for all types. Working pressure doesn't indicate the pressure at which these things will fail, which is typically higher. Just make sure that they're pushed in all the way. Conversely, to remove airlines you can use a box head or (better) crowsfoot wrench to assist pushing in the fitting to release the line. Sharkbite also makes a tool that does the exact same thing for ~$2.
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