3M 9404 is one of the most popular replacement parts for the 3M PG II pneumatic glue gun. We recommend that is you have 2-10 applicators in operation you always have one 9204 on hand, if you have 11-20 PG II glue guns running it's a safe bet to have two repair kits on hand.  See the 3M PG II Glue Gun Owner's Manual for instructions on repairing the regulator.