COMP Cams Valve Seals 520-1 A vacuum reserve canister captures an extra shot of vacuum from your engine for your power brakes. The COMP Cams Vacuum Canisters are for cars equipped with big cams and power brakes. 1 Metal Body Viton Valve Seal for .530" Guide Size, 5/16" Valve Stem COMP Cams Vacuum Canister, Zinc Plated and Polished COMP Cams Valve Seal 5/16 Viton Metal B