Geoff Weeks

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.

Looks more and more like the MV (multi-function valve I.E "buttons") are the problem, tracing the yellow line would help to confirm.

Again, Do you hear air leaking the WHOLE time it is dropping to 80 psi, or just a short time while the button is pushed in? Trace that yellow line back. As you can see, there are two yellow vent lines going through the firewall so it is important to know what valve this one is attached to so as not to replace the wrong part.

I'd be leaning toward the buttons/ MV itself, but without more clarity it is just a SWAG. Trace that yellow line back to where it goes, that will give important info on what/what system is venting.

Just to be clear: When you push the buttons in, it leaks ALL THE TIME until 80 psi on the tractor gauges and then stops and builds back up and doesn't leak after that? OR; does it momentarily leaks BUT the pressure drops to 80 and build? They are not the same.

What is important is when it injects on the engine, anything prior to installing on the engine is just to make sure you are in the "window" where it can be adjusted when installing. May be the Bosch pumps are done different (Robert or American) I don't know, but the various inlines I have done, it was get it on the engine in the aprox correct position, then spill time to the engines timing marks. Inlines that I have done, if the engine gears are properly timed on assembly, then the "double spline" will be in the correct orientation so when the pump is fitted, you can move it in the slotted holes to bring into time for that engine spec. Someone mentioned looking through the big plug on the side, to look at the coupler. I am not going to say more because I don't have enough experience on exactly what you are working on. If it were I, I set the pump so it was at injection #1 and the engine the same, and slide the pump home, then check via spill time, and adjust as required. If I was way off, I may have to shift the coupler spline one notch, but that assumes there isn't a double spline (which has happened to me a time or two) to guide me.

I think fjh covered it one page one. The internal setting up of the pump is done on a pump stand, all you do when putting on the engine is set the timing. I don't know what you are looking to do on the bench?

Well I think your eyes and the view of the timing mark will have a much bigger effect then the differences between spill and flow. There is a device that will trigger a timing light off an injector line, which would allow you to strobe check, then the issue is if there is an advance unit that would be effected by engine speed. I have a injector pipe that is cut at an angle so you can see exactly when the fuel start out the pipe, it is bent so it is almost horizontal, so a bead of fuel at the end of the pipe is visible.