I have had the same freeze problem with my unit. When you simply drain the valve the water remains in there (because of the twisted pathway) and then it freezes. If you add anti-freeze it doesn’t get to this valve because of the same twisted pathway.
When using air pressure you need to step on the pedal and trigger the sprayer that's attached to the same valve until they both run dry. My manual doesn’t mention anything about this procedure either.
I haven’t had a problem in several years now that I blow out the pedal and spray line.
Hap