"Simple air brake sytem question"

OK, I'm sure it's simple,, but.
'03 Pete 1.1 million miles.
Frequently now when I set the tractor brakes to park and have my foot on the brake pedal I get air out of the exhaust port of the Mv3 (parking brake) valve. Doesn't do it every time, but it's becoming more frequent. If I stab the pedal it stops and won't do it again until I charge the system and repeat.

TRLR park valve open or closed no difference.

Pulled the "Mv 3"valve assy. Put about 10 drops of Marvel Mystery Oil in each port and blew shop air (gently) through each port and reassembled, no difference.

Before I spend $117 on a new valve assy. I'd like to get some guidance.

BTW, this valve assy. had air lines with a quick connect type fitting. I just couldn't figure out how to get them apart. For those who would like to know, they have a small collar that you collapse with your fingers and then you "encourage" the air line out,, simple! Reassembly, you simply ram the air line back in and give it a tug
Thanks for any input.

 

^^^^ Welp,, that didn't work ^^^^
Replaced the "R 12" valve" aka anti compounding valve and still the same symptoms.
I did find some grit, sort of, inside the old valve, which is puzzling because the truck has had an air dryer since day one with periodic filter changes.
I guess the next step is the 30/30 chambers. They've never been changed.

 

Thanks Heavyd, hope I didn't imply your advice was wrong.

 

Hey thanks for the heads up. I'll give it a look and report back.

 

Well, it appears the problem is resolved. 2003 Peterbilt 379 exhd.
Here's the rundown,
Problem: air discharging through dash parking valve when parking brakes are set. (yellow knob) Only happened with slight pressure on foot valve.
1. Changed anti compounding valve (R 12) no help.

2. Pulled QR valve, disassembled, cleaned and reassembled. no help.
2a. Slight blunder, I disturbed the O ring that seals the two halves of the valve resulting in a minute air leak. In this particular valve, unless there's a split diaphragm, a broken O ring or spring I can't see much that can go wrong.

3. Replaced RT. 1.1 million mile maxi chamber and the problem seems to have gone away. I never had any air leaks or issues with either maxi's. Ran out of time and had to go to work so the other side didn't get replaced at that time.

4. Replaced LF. maxi chamber to balance things and the leaking QR valve.
So far So good.

I should've started replacing the cheapest parts first! But, I'm an expert for not following my own advice.
My new motto is, "if it ain't broke, I'll fix it anyway" LOL.
I did go on the Bendix website and whoaaa, there's a lot of info there!