If your brakes are spongy on Dot 5, then you have a compatibility issue with some of the rubber parts in your system. No amount of flushing will "restore" damaged parts. My suggestion is to replace the seals and o-rings and use DOT 3 unless you have components that are rated for DOT 5.
The headaches associated with DOT 5 incompatibility far outweigh any performance values on a vehicle that does not see high brake temperatures. Plus, the counter at the Podunk Parts store doesnt carry DOT 5 if you ever have a need to pick up brake fluid on the road.
