You have to do it right.  The check valve is required.  I have two spreaders with exhaust ejector pre-cleaners and I eventually either clean or replace the air filters when I can no longer stand not doing it.  The restriction indicators never come close to the service point.  I realize that a spreader is not usually in the trash and dirt like a combine, but the difference in cleaning time is huge.  I do buy the expensive Donaldson service by restriction air cleaner elements, not the standard ones that you can get at the parts store.