87 Mack RD685p Won't build air

Old single axle Mack had a problem with building air about 12 years ago when it was last parked. Just pulled it back out, runs great. She's brand new practically with 110k miles. Have done all the easy things like replace the governor, air dryer and check the valves in the compressor to make sure they weren't stuck. If we plug the line off the back of the first tank that runs to the front of the second tank, she will build air up to 130 on the one needle on the gauge. However if the lines are ran they way the should be, it only comes up to 12 at an idle and maybe 40 with some throttle. Could the second tank have an internal issue? could those 35 year old air lines have a clog in them. Maybe even a sneaky little leak that you cant hear spitting all our pressure out? Does anyone have any experience with air problems other than the compressor and governor, that's all I can find out about. Just made an account here and don't know if I'm even posting this in the right place. Any advice appreciated.

 

FIXED!!!! to anyone who is in a similar position, It was an original rubber line, coming off the front of the big air reservoir going to the other smaller one. There was no audible leak or air coming out. We replaced compressor, governor, and only noticed the problem out of coincidence. These old rubber lines can have a bad air leak but between the two layers the sound can be dissipated and unnoticeable.