Dust filter unit for canisters

A dust filter unit for a canister, comprising:

• a housing having a first wall and a second wall partitioning an interior of said housing into a first chamber, a second chamber, and a third chamber;
• an inlet port formed in said housing and opening into the atmosphere and into said first chamber;
• an outlet port formed in said housing and opening into the atmosphere and into said third chamber;
• a communication port formed in said housing and opening into said second chamber and leading to said canister;
• a filter accommodated within said first chamber;
• a first one-way valve arranged within said second chamber, said first one-way valve having a valve bore formed through a portion of said first wall partitioning said first chamber and said second chamber, and a valve element disposed to open said valve bore when a pressure within said first chamber exceeds a pressure within said second chamber by a predetermined amount;
• and
• a second one-way valve arranged within said third chamber, said second one-way valve having a valve bore formed through said second wall partitioning said second chamber and said third-chamber, and a valve element disposed to open said valve bore when said pressure within said second chamber exceeds a pressure within said third chamber by a predetermined amount.