Geoff Weeks

It looks like you are missing part of the valving on the MV-3

we're going from bad to worse here. I think it is time to call in professional help or someone who knows air system to put another set of eyes on the problem. It is likely something simple, crossed lines, something left dis-connected etc, but before you do more, we need better info then you can provide. Not you fault, you just don't have the experience needed. Mecho,Joey Mack or I can't "see" what is happening and how the valves and air system is from this end of a computer screen. We are working blind, and you can see but don't know air systems well enough. Not a good chance of success. Do you know anybody with more experience that can come and help you?

With the air lines on the trailer hooked up, push the red in and hold it in for 15-30 sec, it should stay in.

Red button will not stay in if the red trailer line is open to air. You need to put a dummy gladhand on to test the tractor. I think, according to what we have read, that it does eventually stay in when hooked to the trailer after holding in long enough to charge the trailer. This is normal, the tractor is not the problem. In all my years on my own equipment I only changed one MV and that was because I couldn't get the gasket to repair the old MV-2 on the truck. They don't go bad that often. I since got a new gasket for the MV-2 and re installed it, kept the gaskets in stock because all 3 of my truck had the same valve (which was why I re-installed, the replacement was a Midland and the rest of the fleet was Bendix MV-2)

Ok, truck is fine, problem is trailer is leaking off. Concentrate on the trailer. chock wheels, start truck allow to build pressure push in buttons a fully charge the trailer until the compressor shuts off (dryer vents) then turn off the engine without disturbing the buttons and watch the air gauges for the 3 psi/min max drop. If it is dropping fast it is a big leak and should be easy to find, if it is dropping slow it will take more work to find.

Ok, if I understand you correctly, the problem only shows when you are connected to the trailer. Please confirm this is so. If the above is true the trailer is the problem, not the tractor. A trailer that completely bleeds off all stored air pressure so every tank in the trailer is at 0 psi, will require briefly holding in the red button when it is 1st charged until the supply line to the trailer reaches the "autotripper" pressure in the valve. This is normal operation. The valve is supposed to automatically pop out if the line pressure is below 25-40 PSI "why did this just start" you may ask? Well you have a leak in the trailer system that is bleeding off all the pressure. Normally it is common for a trailer to bleed down when parked, esp air ride trailers, but there is a valve that is supposed to cut the air to the air suspension and save the air pressure below about 55 psi for the brakes. So how do you determine if you have a big leak, by the DOT pressure test I wrote about earlier DOT pre trip says this is supposed to be done with the pre trip inspection, or by measureing the time it takes for the compressor to cycle back on (blow off to blow off time). I freely admit that I don't do the DOT pressure check on pre trip but rather assess the condition by the duty cycle of the compressor. They both are a measure of how much pressure leaks off in a given time. The DOT method is the one you may be tested against in a level 1 DOT inspection, my method is just a easy way for me to assess the condition of my air system while driving. So called silent leaks can be hard to find, it takes going over everything with soapy water in a spray bottle, looking for bubbles. Sometimes a piece of hose held to your ear can mask surounding sounds and help narrow in on a leaky valve. Braided covered rubber hose can leak along its length and not make a sound but will bubble the entire length when sprayed with soapy water.

These were from page two, It wasn't clear to me or to Mecho that you had disconnected the trailer at that time. We are not mind readers. Your admission that you don't know what the system pressure in an air braked truck is, shows you need some guidance from someone looking over your shoulder until you learn the very basics of air brake systems. Operating pressure and how to check for leaks that can't be heard are very basic skills needed to work on the system. They are also part of the pre-trip portion of the CDL test.

The problem is: he doesn't know enough about air brakes to pass the pre-trip portion of the CDL test, there seams to be no way to get him to clearly answer or follow the directions. Mecho in the 1st 2 replies was trying to get him to separate the truck from the trailer air system to see which it was in. It wasn't until much later on, that he did disconnect the trailer, by then we were well down the rabbit hole because he didn't do what was asked by the 1st guy who tried to help. I mistakenly assumed he had followed Mecho's advice and was digging deep into the truck system, when in fact he hadn't and so I sent him on a goose chase based on what he said, not what he did. Yes, using an inflatetor hose (depending on length) is more or less plugging, but by that time the damage was done. A person who doesn't even know the correct operating pressure of a trucks air system, should not pass a CDL test, never mind work on truck air brakes. Ignorance is not a sin, everybody starts out not knowing stuff, however if you ask for help and are given clear instructions, either follow and report or admit you don't understand or know how to do what is asked. The lack or clarity in his responses likely cost him the price of an MV-3 and the time it took to install. Not knowing the working pressure of a truck air system or how to look for leaks (Something required in a pre-trip) is a clear indication that he shouldn't be working on the brake system without some qualified help right there with him. It is no skin off my nose (or as we used to say in the repair shop "My truck works") but people can get hurt or killed if a defective vehicle is on the roads. Again, everybody has to learn, but you have to be willing to admit your ignorance and ask questions if you don't understand what people like Mecho are asking him to do. Mecho's 1st post was to break the problem in half, the best and most basic diagnosing action. Ignoring it and not clearly saying he didn't do it, wasn't accepting help given freely. You can't learn if you are not open to it.

I wasn't criticizing your question, sorry if it came off that way I didn't mean it to. Please accept my apology. I was just saying, that high cut-out pressure wouldn't cause the problem he was having. He may have multiple problems, esp if the pressure is that high, but it wouldn't be the cause for this particular problem. Because of the lack of clarity from the O/P all the way through, I think focusing on one at a time.

F@#$ the pressure for the moment! Will it pass the DOT test? It is supposed to be done at pre trip, but no one ever does, including myself. I use the "duty cycle" time of the compressor while running down the road. If my compressor has high duty cycle I know I have a leak even if I can't hear it. If you can't see 3 psi on the gauges, go 2 min and look for a 6 psi drop, or 4 min for a12 psi drop. If you can't tell 12 psi difference on a gauge reading either you need new gauges or new eyes.

You can check another way: DOT requirements are: run the truck until the compressor blows off, with the brakes released (chock the wheels) shut off engine. It can leak no more than 3 psi/min per res, so for a truck with a primary and secondary air system, the truck can only loose 3 psi/min from each tank. When the trailer is connected you still can't leak more than 3 psi/min with the brakes released.

That is the problem isn't it? We can only respond to the info we are given, not the info that is not conveyed.

If the tractor works correctly when the trailer is disconnected and the red line plugged (you have to plug the red line to push in the red knob) then yes you have to start looking at the trailer. AGAIN, how long does the compressor go between cycles? Best truck I had would go 45 MINUTES, between cycles running down the road, with an air ride truck, air ride trailer, and a full load. Worse truck I had ran 12-15 minutes. If yours can't do that you have leaks to sort out.

More or less you are correct, he needs to wait for the system to fill, but the question remains "why is it loosing all its air?" Even with air suspension, the protection valve that feeds the suspension should close at no less than 55 psi.

Maxi back feeding is often blamed for the problem when it isn't them at all. When a maxi leaks internally between the spring brake side to the service brake side, it then goes back up the non pressurized service brake line. 1st thing it comes to is the open quick-release, either a stand alone QR or a QR that is part of the relay valve, Assuming that is defective and stuck closed, it would travel back up the service brake line where it (if we are talking trailer problem) come to the double check valve between the Johnson bar and the treadle valve, both of which would be open to vent without being applied. If it were the tractor and the leaking maxi wasn't on the side that feeds the double check, it would still have an open vent at the treadle valve. A relay valve can leak internally so res pressure is fed back up the service line, but it still would vent from either the treadle valve or the Johnson bar valve exh port. In both cases the truck will not be able to build air with the brakes off but will if either the Johnson bar or the foot pedal is applied.

I don't see a "backfeed" from a leak causing the problem. For one there are too many quick-release valves that remain open when brakes are not applied, for it to backfeed past all these isn't one failure but many. 2) the problem is one of flow, it exceeds the ability of the truck to supply in the short term. One quick way to see if the combination is leaky, is how rapidly the compressor cycles off and on. It shouldn't be more than once every 15 min when running down the road, not applying any brakes. That may be hard to test in Conn traffic, but if sitting in the yard with the brakes released it shouldn't cycle quicker than 15 miniutes. Time the "blow off" from the air drier, and time to the next blow off. Less than 15 min and you have work to do. When you hook to a trailer with a completely empty air system, it takes a large volume of air, esp if the trailer has many tank or very large tanks like a belly dump. With the red trailer line plugged off at the "gladhand" the total volume of the lines from the button, through the TP valve and to the plug at the end of the trailer hose is very small, it will quickly pressurize above the tripper pressure of the red knob valve, and it will stay in. With the trailer hose not plugged and not connected, it will never stay in because the pressure can't build in the line. With it connected to a trailer that leaks off all its air overnight, it has to fill the trailer until it is above the tripper pressure on the valve. In your case, it is taking a long time and there is some resistance but the trailer need such a large volume of air, the the pressure at the red knob is right around the tripping pressure until the trailer is more pressurized. To put it in the shortest words possible, you have a trailer problem not a tractor problem.

You can't test without the red line connected unless you plug it off, otherwise air should just escape and the red button will not stay in. This is what DOT does on a level one to make sure the buttons pop out.

The quickest way to diagnose, it is split the system in two, by disconnecting the air lines from the trailer, you separate the tractor air system from the trailer air system. Had you done that (and I thought you had) you wouldn't have air leaking out the yellow line, and I wouldn't have sent you down the wrong (and expensive) road. Then you would know the trailer is the problem, and we can start working on finding the problem on the trailer.

Mecho told you on page one to block off the trailer lines, I assumed you had. It is looking like the trailer is dumping all air (It isn't a belly dump with a huge air tank is it?) If the trailer requires a full fill it can take a lot of air. You are moving too much air into the trailer line, more that the 3/8" line can supply at the volume needed to recharge the trailer. Once the pressure builds in the trailer, the volume decrease and the pressure at the valve rises above the tripper pressure. Important to follow all the points on suggested or VERY clearly state you did not.

Only thing I am coming up with is extremely unlikely. BOTH primary and secondary supply lines to the valve are somehow restricted (Crimped, bent or plugged with something) so a small amount of air can pass, when the yellow button is in, but when a large amount of air needs to flow (to charge the trailer) it can't supply the volume of air needed and the pressure drops at the valve to around the tripper (25-40psi) point, which is above the tripper point on the yellow, and the tripper starts to vent, once the pressure rises enough it stops, and charges. This is extremely unlikely. I have never seen it, but it is based off how the system is supposed to work. If only one system supply line was crimped, the shuttle valve should switch to the tank that can provide supply. Since you replaced the MV-3 it shouldn't be the valve is the problem, but I am at a real loss to explain why the only indication would be venting from that yellow line. Anything like internal cross leaking on the TP valve should show venting at other places.

Try pulling down on the "Johnson bar" (trolley valve) for the trailer brakes, BEFORE you push in the buttons, does it still leak when the buttons are pushed?

Here is the manual on that valve: https://www.bendixvrc.com/itemDisplay.asp?documentID=6599

Ok, so yours are the only eyes and ears there, so we need to be clear, when you say it keeps venting until 80 psi, BOTH primary and secondary tank pressure falls to 80 psi? Is it only one that falls and the other stays at 120? No other leaks? The output should go to the TP valve back on the frame somewhere, No leak? Does it do it when only the yellow is pushed in? (Bobtail position)? No work was done on the system before this problem showed? Are you pushing and holding in for a few seconds (for the pressure to hold them in) or just tapping them in?

https://www.zoro.com/bendix-module-control-brake-mv-3-028-in-dia-14-npt-800518/i/G111330826/ Here is the valve If you pull the buttons off and remove the recessed screws you should be able to pull the cartridge(s). edit: the one I linked to was just for generic picture, they come with different ports, some are threaded some a push to connect, get the one that matches what you have. Should be a number on the Bendix tag I can see on your picture.

Yep, that is the MV valve. Bendix, I don't know (can see from pictures) if it has the cartridge valves that can be replaced separately or you have to change the whole thing. Most change the whole thing because that eliminate a small internal crack in the plastic housing from being the problem and not the cartridge valve itself.