Air brakes- R12 valve leaking through exhaust

Hi everyone, I have a 98 fl70. I’m having some issues with rapidly loosing air out of the R12 valve exhaust port. This is a single axle truck. I’ve replaced the r12 valve, both brake chambers and the rubber diagram on the QR valve. The QR valve was the original issue, with air leaking from its exhaust port so I replaced the rubber disk, which had multiple holes, and the brakes released fine. Drove it about 40 miles when the driver side brake started locking up. Then I noticed air coming from the exhaust port of the r12 valve so I replaced it and the brake chambers, but still losing air through the R12 exhaust port. The air will build just fine when the parking brake is applied, but pressure will rapidly fall when released. Could it still be the QR valve? Should I replace with a new QR valve?