Wish I had found this thread several years ago when I was resurrecting my mom’s solar. I ended up replacing the collector loop pump and draining/recharging the loop in order to get it functional. She doesn’t rely on the solar hot water, but does have two solar radiators to heat her living areas. I was having to recharge the collection loop annually, but my son found a leak in one of the collectors two years ago. We bypassed that collector (1 of 3) and pressure seems to be holding.
Now there is a rattling sound coming from the Sunstream box that make me think a bearing has gone bad or an impeller grenaded. Open to suggestions.