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You can hook the vacuum gage in anywhere you want. It is best to go to the end of a hose [so you don't have to cut into a line]. The gage you bought should have come with some little line adapters and an extra length of tubing so you could hook up however you need to. To see what vacuum your engine is capable of making, disconnect and seal off all lines coming from the fitting on the intake manifold (usually a 2-way or 3-way fitting) then connect your vacuum gage to just one of the ports. Start car (best to be at normal operating temp), set at idle and read the gage. Record that reading (or adjust the carb and distributor for max. vacuum) and re-attach the original lines. Now wherever you hook the gage in, you can compare the results to your "baseline" reading to see if you have a significant leak or not.
