Could be anywhere since these trucks are so old.
Since you've "tested" the pump and it shuts off as it should, you can connect sections to it and test it after each one to see if it leaks.
Your best bet is to go buy an el cheapo vacuum pump (Amazon, Harbor Freight, etc.) so that you can test the system with your hand instead of the vacuum pump. These pumps were not meant to run continuously (and are not cheap to replace). You will also be able to see actual vacuum and where it holds at.
I'm not sure of the HVAC system but it only takes about 10" to actuate the hubs and 6" to release the hubs (or vice versa...I don't remember which).