Supply air system

[revlimited]

Hi truck is a 2007 freightliner Columbia. Has an air dryer connected to the primary tank. Almost like an AD-IS. When both primary and secondary tanks reach 135 psi, the purge does not burst air but slowly leaks out air for 20 seconds. While this is happening both tanks drop to 125 and it starts building again back to 135. I have replaced the white check valve, spring and o rings with bendix kit 5004052. The check valve which I removed was broken.

So whats the problem? Why is the truck losing primary and secondary air when its purging?