Before I attempt to change the powerhead on this valve i wanted to ask just one simple question. Should the valve be left in the auto position or locked in the manual (open position)?? I'm assuming that the entire actuator will end up coming off the valve body when i remove the 2 screws for the powerhead

I have an oil-fired boiler and with 3 Taco 571 zone valves and 1 Taco 007 pump. There are 2 heat zones and 1 zone valve controlling a Super Stor DHW indirect holding tank. The 007 pump is located on the return line just downstream from the return manifold (close to the boiler). After 22 years, the zone valve to the Super Stor is going bad. Definitely not the motor - I can remove the motor/head completely and that 571 will still allow flow if a heat zone is on. The only way to stop this flow is use the SS shutoff at the drain. Might be a spring, or something cracked inside the valve, but I definitely need to open it up and try to swap out the hardware. There are no isolation shutoffs at the zone valves, so given that I have to drain the boiler, I'm looking at the refill job and having a problem figuring out how to avoid getting air trapped in the return side of the piping. I know how to bleed zone-by-zone, and have done that job in the past, but the boiler was basically full at the time. The system has the standard auto-fill valve, which I know will fill the boiler and (one zone at a time) push water up the boiler supply pipe, through the zone valve, downstream piping, and eventually out the zone drain. So that takes care of getting all of the air out, EXCEPT that there's a return line from the Super Stor back to the boiler and also the return line back to the boiler from the common return manifold (downstream from the zone drains). I don't see any possible way for these 2 return lines to get air purged and water in during the purging of the zones, because those sections of piping become isolated from the purge water flow. And I'm expecting that when I open up the system, all of this trapped air will flow throughout the entire system and result in a big full system air purge effort (or maybe even more than one). I'm trying to figure out how to avoid that and get those return lines purged along with the rest of the system. I'm thinking the only way to get these lines purged would be to use the fill valve pressure to first push water in reverse flow up the return piping until it flows out the zone drains. After doing that, I would close the zone valve shutoffs and perform the regular forward flowing zone fills. But I'm not sure about the pump. The 007 pump doesn't have the IFC, so I'm thinking it should allow water to flow through it in a reverse direction, but can't find anything to confirm that (with no damage to the pump). Has anyone been through a similar thing in the past? If so, is my idea above basically correct, or am I missing something about this whole boiler refill process? Maybe the air can't be pushed out of the return lines like I'm thinking, and a follow-up full purge will always be necessary. Thanks in advance for any insight and advice on this.