Leaking slide valve?

[cmcq]

I run my pump at night on a timer. The last couple of nights, I woke up to the sound of big air bubbles coming out of the returns. I figured it was air getting in the system somewhere, then sitting until the pump fired before getting pushed into the pool. I re-tightened everything - the pump basket lid, plug in the bottom of the pump basket, plug in the pump, the band around the filter, etc. and didn't see any water leaking out. I let the pool settle for a few hours, then checked it out.

I heard bubbling inside the filter (DE) and hissing from the slide valve used to backwash. I moved the valve handle back and forth and open/closed to try to stop it, but it kept hissing and the bubbling inside the filter continued. I tightened the screws on the valve handle and the hissing stopped, but not the bubbles in the filter.

My wife suggested I run the pump while looking for leaks (how did I not think of that?). With the pump running, I saw water leaking from the bottom of the filter so I re-tightened the drain plug (also forgot to do that the first time). That leak stopped, but now there is water leaking from where the pipe first comes from the filter into the slide valve assembly, and around the handle of the slide valve.

So, I suspect this is where the air is getting in when the pump is off, but how can I be sure and how can I fix? Do I need to replace the whole valve or are there o-rings inside I should check/replace? How do I know which parts to get? Until it is fixed, can I run the pump if I bleed the air out of the top of the filter (the leak is very slow)?

Thanks
Charlie