Monash repair kit for 1/2" 30, all patterns, radiator traps including bellows, plunger, and seat. Thermostatic bellows are a sealed liquid filled capsule that closes to keep the steam contained in the heating apparatus, and open to allow the discharge of air and condensate when it cools. They are used in either radiator or fin tube heating applications. When steam is present the heat expands the fluid within the capsule, causing it to push its outer plunger into discharge seat sealing the steam. As the steam cools it causes the liquid to contract which raises the plunger allowing the air and condensate to discharge.