MV3 Valve leaking

[Hunter013]

2005 Peterbilt 378

MV3 valve leaks out of exhaust port when tractor brakes are set. Intermittently leaks when tractor brakes are released, but stops if I hit the foot brake.

Changed the MV3 valve, no change. What else should I look for?

 

[Hunter013]

I also took off the delivery lines for tractor and trailer and there is slight air pressure coming out of the delivery ports.

 

[Truck Shop]

I also took off the delivery lines for tractor and trailer and there is slight air pressure coming out of the delivery ports.

If there is pressure at the delivery ports of either the MV3 valve or the Foot Valve--you need
to trace those lines back--because your getting back feed from a anti-compounding QR1C
valve or something along those lines. Hell even a brake can leaking internally can cause
a back feed pressure to delivery.

 

[Hunter013]

The air is coming out of the delivery ports on the valve itself. So the only lines hooked up are the 2 supply lines. Do I trace those back? Are they both supposed to have constant supply?

 

[Truck Shop]

With buttons out?

 

[Hunter013]

Correct. Button’s pulled out

 

[Truck Shop]

With supply lines in correct positions on the MV3--and delivery lines removed-there should be
no leaks through delivery ports--bad MV3.

 

[Hunter013]

Put a bendix valve in it and so far works great. The last 2 we had in it were automann brands.

 

[Truck Shop]

Put a bendix valve in it and so far works great. The last 2 we had in it were automann brands.

I'm not fan of off brand air valves--I either use genuine Sealco or Bendix-just for the
same issue you had. You don't want to replace a set of dash valves twice in a
Freightliner Classic cab-it's not a good day.