I used my Latex brush for Zinsser's BIN Primer/Sealer which suggest either Ammonia (50:50 with water) or alcahol for cleanup.
I didn't have any ammonia so I used undiluted Fantastic which seems to work well too (perhaps even better than ammonia).
In any case, I noticed that the Amonia/Fantastic not only cleaned out the primer but also softened up years worth of caked on Latex paint. After several hours of additional soaking and intermittent combing with a brush comb, my old brush was restored to like-new condition.