Power washing my block foundation to remove old peeling, flaking paint has left pock marks and some loss of mortar in joints. How do I repair and then what type of primer paint should I use? (Don't know what the original paint was-oil based or latex). I'd appreciate your help. Thanks.

You can patch any holes or missing joints w/ Sakrete mortar mix then you can paint it with Drylock or a paint recommened for foundations.

You can also parge your foundation (thin coat of mortar) with a parging mix (Sakrete) then paint over the parging when dried.

Another option is to use insulated foundation panels. Menards, a regional depot big box store, has advertised them for sale. I think they are 2' x 4' panels that can be cut with a knife.

They are installed using adhesives and /or mechanical fastners.

Regardless of the option you choose, make sure that you repair the mortar that is missing.

If you decide to follow SkiBergers excellent advice, allow it sufficient time to cure before you paint it. And make sure that you use paint designed for masonry. Regular housepaint doesn't breathe the way masonry paint does so it is more like to chip, blister and/or peel.