Then maybe get a pro to look at it. I suspect you simply have your o-ring that needs lubed and the collar ring tightened. This all depends on if you can remove the m-port or not to get the m-port off and re-lube/reseat the o-ring. Mine was permanently plumbed so I couldn't remove it without cutting and installing unions - unions allow me to disconnect the PVC pipes from each other and lift off the m-port. Suction side unions are normally a bad idea, but you're all on the pressure side at your multiport.
There comes a point where if you're not comfortable with it, you call a pro. Not joe shmoe pool store, but somebody who installs pools and plumbing. What could take you all weekend, they can do in an hour or two as well as a warranty/guarantee.
Keep in mind I'm a newbie, but I've learned TONS from having to do repairs on the pool plumbing from a house I bought last year with the pool
-edit-
Age of multi-port and filter? Watching your pressure gauge? o-rings do need to be replaced or lubed from time to time (many years).