I'm a bit confused about the bubbles. You think you have a leak in the suction side that's bringing air into the pump and filter? Those are hard to find. I've read about people having success finding suction leaks by smearing shaving on the fitting and looking for it to get sucked in. One poster here this year stopped a leak with shaving cream - go figure. if you know a threaded fitting is leaking and you don't want to open it, you could try smearing some non-hardening PVC compatible thread seal on the end you can reach.