Air leaking from blue line,
 

I have a century class, lately its leaking air big time (from blue line behind bunk ), every time i step on brake padle ( bob tailing ). Air even leaks when yellow and red dash buttons are pulled out and I step on brake padle. Any idea what might be causing it... Thanks

Now that sounds like a tractor protection valve problem.........

Thanks for your response.

Yellow dash valve should have nothing to do with it, BUT, it sure sounds like a faulty Red Dash Valve.

Of course, they are not a single unit anymore, they are a module. so If you swap one out, then the other goes with it.

There is no air to the Tractor Protection Valve UNTIL you push in the red dash valve. Until then, it works like a seal. Once you put air to the Supply (red) then it moves a slider which enables air to come out of the service (Blue) when you make a brake application.

Could be the TP, but more than likeley it is the Dash valve.

Do you hold the trailer in RED, when you dynamite the tractor Yellow, and then pull the Red to set the trailer brakes. If you do, there is a really good chance that is what caused it.

Proper procedure for setting your parking brakes, Tractor with Trailer attached is as follows

Pull yellow, red will almost always follow.

Even if you need Air in the trailer for some reason, you need to set Yellow (red follows) then push the red dash valve back in to release the parking brakes at that time.

I see it often where guys come in and hold their thumb on the Red button while they pull the Yellow button with the middle two index fingers, then pull the Red out.

I have module dash valve set in my shop that I use to show what happens when you do it. They eventually leak.

The manufacturers sent out notices years ago that it is not advisable to hold in Red, while you dynamite Yellow.

I'm gonna vote for the tractor protection valve.

I'm not understanding how dash-valve delivery air (trailer supply from red button) is getting to the blue line (trailer service from treadle) without a TPV internal leak? Both the supply and service air flow through the TPV, but should be isolated from each other.

I had a small crack in my blue air line right off the glad hand. every time i applied the brakes i would get a swooshy feeling on the pedal and a noticeable psssh noise coming from the hose. took awhile to find the crack because it was pretty small. i also get a air leaking noise from the yellow valve in the truck when its pushed in. its a constant light shhhhh noise. not until recently did i come to find out if i just push it sideways a tidge the air sound stops. it was driving me crazy actually.

I rethought this. and then traced it at work on my Air Board.

If, air is leaking through the red dash valve, activating the TPV, which is then allowing air to move through the blue line, there would also have to be air loss on the red line. (Bobtailing)

If, air is leaking ONLY through the blue line, then it would probably have to be the TPV is leaking on the blue side.

I concur, sounds like a TPV to me also.

In other words.

Red dash valve is out, and air is leaking through the red glad hand, the culprit is probably the Red Dash Valve. When you make a brake application, air would also come out the Blue glad hand.

Red dash valve is out, a brake application is made, and air is leaking through the blue glad hand ONLY, the culprit is probably the TPV itself.

Red dash valve is IN (NO BRAKE APPLICATION), air is coming forcefully out the red dash valve, and there is air coming out of the blue glad hand, then the leak is internally in the TPV, between the Supply and the Service side.

When you teach air brakes, the TPV and the Spring Brake Modulator are the most fun to teach.

Thanks to everyone for the input, I changed the tractor protection valve. And it solved the problem. Once again a big thankyou to all, I also came to know that this is a out of service defect if cought at inspection station.

Even if the red dash valve leaks out to the TP control port, there is still no air on the blue line unless a service brake application is made.

So, in order for air to leak out the blue line, you would require a 'second' leak; the first leak is through the red dash valve, the second leak through the foot valve (where the blue air comes from), and it's VERY unlikely that foot valve air would leak from 'supply' to 'delivery'.

And if the foot valve was leaking across from supply to delivery, the tractor service brakes would be applied.