Compression Ignition Addict
Join Date: Nov 2008
Location: Lumberton, TX
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The majority of the time it will be one of either two things:
1. The vacuum line to the hub has either been knocked off, dry-rotted, etc. right where it attaches to the hub itself. It's just a rubber hose slipped over a little nipple on the hub so if that's it you can probably just cut the bad section off and slip it back over.
2. One of your seals has an internal leak. If that's your problem you'll just have to replace the seals in your hubs. They're not very much money. If number one doesn't work out borrow a vacuum tester from someone and try to pull a vacuum on each hub and see if it will hold.
As far as testing the pump you should be able to just pull the vacuum lines off of it, plug the ports and see if it will quit running. If it does you can just put the lines back on one at a time and see which one causes it to keep running.