TM 10-3930-659-10 3-10.  CHECK AND FILL HYDRAULIC SYSTEM RESERVOIR (Con’t). 3-11.  CHECK AND CLEAN ENGINE PRIMARY AIR FILTER AND UNLOADER VALVE. WARNING If NBC exposure Is suspected, all engine air cleaner air filter media should   be   handled   by   personnel   wearing   protective   equipment. Consult your NBC Officer  or  NBC  NCO  for  appropriate  handling  or disposal procedures. a.  Check and Clean Engine Primary Air Filter. (1)  Loosen  wingnut  (1)  and  remove  air  cleaner cover (2). 3-17