Each pool is unique in operation and your results may vary. First, at low speed the GPM is 1/2 of the high speed value not 1/2 GPM. So if your normal flow is 70 GPM, at low speed it will be 35 GPM.

When a filter gets dirty, the pressure ahead of the filter goes up and the pressure after the filter goes down. So, the problem you are experiencing could be due to this pressure decrease when the filter is dirty. You do not have enough pressure to fully prime the solar pipes. It sounds like you are waiting too long before cleaning your filter.

Another reason could be that your vacuum release valve is not operating properly but difficult to prove. Normally, they only require a few PSI to close and negative PSI (vacuum) to open. So if it is not closing and you have enough pressure after the filter (hard to determine) then it could be a malfunctioning valve.

Of the two, my guess would be the first problem. So the solution is to keep your filter clean. With a clean filter, there should be no problem with low speed but I would need to know the pressure readings that you currently have.

Pump model, HP, Service Factor
Clean Filter Solar Off PSI
Clean Filter Solar On PSI
Dirty Filter Solar Off PSI
Dirty Filter Solar On PSI

With these numbers, I can assess if a two speed would work for you.