I think I would add 3-4 valves to the system so you can isolate the problem area. Then start the system up with all the valves closed off. Open the valves one at a time until you locate the section which causes you to completely lose vacuum. I don't have any experience with vacuum on maple sap collection systems, but have battled vacuum problems in industrial applications for many years. You might also use an air compressor to pressure the system to a very low pressure i.e. less than 5 psi and walk the lines listening for the leak. Good luck.