Johnny Rock

I know this post is five years old. I just had the same exact problem on my 1994 Mack RD. After two days replacing parking brake valve on dash, relay valve on frame and troubleshooting, no results. I had no choice to search the internet. I am so lucky to of found this posted long ago. I am done replacing that double check valve and everything is back to working properly. I want to take the time now to thank Raindog. Thank you Raindog for sharing ! ....You the Man !