Series BST/BLT/BAT Heavy duty, million-cycle designs with oversized isolated springs and pistons, visual indicator windows and the patented Fail-Dry early warning vent. Offered in a wide range of sizes and materials; configurations include fail-safe spring return, air x air operation, or line pressure to open or close. With limit stops and other options. When it absolutely has to open or close bubble-tight, these big robust valves deliver.