Air Pissing out

[Geoff Weeks]

Here is the manual on that valve:

https://www.bendixvrc.com/itemDisplay.asp?documentID=6599

[nbone28]

20 minutes ago, Geoff Weeks said:

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?

Yes both tanks fall to 80PSI. I put in the new valve, hooked up the airlines, went to go test it out and the initial drop after pushing in the trailer air supply was 90psi in both tanks. After releasing the brake the air was leaking out that yellow airline still. Went to go test it again and the valve won’t stay pushed in. I unhooked the emergency line from the trailer and hooked it up to my airline to air up the tires and had no problem with the button staying pushed in when hooked to that. I go back to hook it up to the trailer and it won’t stay pushed in. 
 

It doesn’t leak when pushing in the yellow button by itself. 

The only recent work was the foot pedal valve, that I replaced about two months ago. 

[Geoff Weeks]

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?

[Geoff Weeks]

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.

[mechohaulic]

you haven't separated the trailer from the tractor YET !!! take the red/ blue lines OFF the trailer and cap them. using a plugged glad hand or holder mounted on the pogo stick. THEN PUSH IN THE RED BUTTON TO SEE IF YELLOW LINE LEAKS . a faulty trailer valve /// brake can will return air to THE TRACTOR !!!!!

[Geoff Weeks]

33 minutes ago, nbone28 said:

. I unhooked the emergency line from the trailer and hooked it up to my airline to air up the tires and had no problem with the button staying pushed in when hooked to that. I

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.

[Geoff Weeks]

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. 

[nbone28]

1 minute ago, Geoff Weeks said:

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. 

I’ve tested it without the airlines hooked up to the trailer, there’s no leaking when you go to push in the trailer air supply and when you go to release it. When I hook it back to the trailer and test it, that is when the air is leaking out of that yellow airline. The problem has to be with the trailer. It’s a flatbed trailer that’s got a lift axle on it. That MV-3 valve was going bad either way it seemed like. Ended up being $211 for a new one. 

[Geoff Weeks]

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.

[Mark T]

Can't you make friends with someone to try a different tractor to the trailer or your's to  a different trailer ?   Narrow it down some.

[nbone28]

10 minutes ago, Geoff Weeks said:

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.

I had the red line connected to my airline setup to air up the tires. It was not hooked up to the trailer. When it was connected to this I was able to push the trailer air supply in and it would stay pushed in. When I went back to it hooked up to the trailer the button will not stay pushed in. 

[nbone28]

5 minutes ago, Mark T said:

Can't you make friends with someone to try a different tractor to the trailer or your's to  a different trailer ?   Narrow it down some.

I do, they just don’t live anywhere “close” by where I park my truck. 

[Mark T]

Just now, nbone28 said:

I do, they just don’t live anywhere “close” by where I park my truck. 

May need to sweet talk someone into it (lol)   Little hard going from descriptions in text, but from what I'm reading if it's not the valve assembly (parking brake s) it's likely a bad maxi on the trailer.  I only mentioned the foot valve cause it looked fairy new. The yellow in the first picture looks to be an exhaust  line, and I don't see lots of goo drooling out of it. Often when valve after valve fails the air dryer hasn't been serviced in a while.  A maxi ???   they can fail for no particular reason and bleed back.. Might even be more than one.

[Geoff Weeks]

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.  

[Joey Mack]

So its not as simple as: he needs to wait for the trailer tanks to refill and then drive on ?? Occasionally i pull a detach trailer, and when I pop the brakes, the trailer dumps air, then when I  release the brakes, i sit there for ten seconds till the suspension comes up, and off I go.  This combo does not lose air when doing my air pressure/loss pre trip.

[Geoff Weeks]

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.

[nbone28]

17 minutes ago, Geoff Weeks said:

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.  

Where do I start with the trailer? I don’t hear any air leaks coming from the trailer. 

[Geoff Weeks]

4 minutes ago, Joey Mack said:

So its not as simple as: he needs to wait for the trailer tanks to refill and then drive on ?? Occasionally i pull a detach trailer, and when I pop the brakes, the trailer dumps air, then when I  release the brakes, i sit there for ten seconds till the suspension comes up, and off I go.  This combo does not lose air when doing my air pressure/loss pre trip.

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.

 

[Joey Mack]

He said it drops to 80.. if he is releasing the trailer before blow off pressure, then I say. No Sh!t...  Anyway, im not in the seat to see it. So I am likely out to lunch..  

[Mark T]

Might be moving enough air into the trailer the parking brake can't decide whether to pop or not. Might not be actually bleeding back.

[Geoff Weeks]

3 minutes ago, nbone28 said:

Where do I start with the trailer? I don’t hear any air leaks coming from the trailer. 

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. 

[Geoff Weeks]

1 minute ago, Joey Mack said:

He said it drops to 80.. if he is releasing the trailer before blow off pressure, then I say. No Sh!t...  Anyway, im not in the seat to see it. So I am likely out to lunch..  

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

[Geoff Weeks]

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.

Edited 10 hours ago by Geoff Weeks

[nbone28]

11 minutes ago, Joey Mack said:

He said it drops to 80.. if he is releasing the trailer before blow off pressure, then I say. No Sh!t...  Anyway, im not in the seat to see it. So I am likely out to lunch..  

That’s with the tanks full, I let the tanks fill up and on the initial release they drop to 80psi. 

[Joey Mack]

What is the air pressure on the gauge when the drier blows off ?