air dryer leak.

I have a 2006 Volvo 780 with a VED 12 that has an air leak that is driving me nuts.
The dryer started leaking out of the purge valve so I replaced the dryer, I had a new air dryer from a 770 so I put that one on so I could get going. that stopped the leak but when I drove it the brake was using a lot more air on the app gauge, it worked so I drove it till I had time to fix the old one. The 780 had a purge line going to one of the tanks, the 770 dryer did not so I blocked the line off. I had some time now so I put a new purge valve and turbo valve in and put the dryer back on. It still leaked from the purge valve so I put on a new air governor and it still leaked. I drove it around to see if the brakes worked better. The dryer purges every 4 minutes when driving every 2 when at idle but the brakes work much better and use half the air on the app gauge. When the dryer purges I get the blast of air then I get a smaller leak that will leak down till the air compressor kicks on then it stops, if I turn the truck off just after the purge it will leak down to 100 psi then stop. What am I missing, is there another valve I need to replace or is the whole unit bad?

thank you for any help you can give.

 

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