What is the purpose of the one-way check valves in an air brake system with multiple tanks?

ONE-WAY CHECK VALVES (or check valves) are installed at various points in the air brake system to enforce unidirectional airflow. PRIMARY PURPOSE — TANK ISOLATION: When air flows from the compressor through the supply tank toward the primary and secondary service tanks, check valves prevent air from flowing backward (from service tanks back toward the compressor or supply tank). This isolation means: if the supply tank develops a major leak, the service tanks retain their pressure; the secondary circuit cannot be drawn down by a failure in the primary circuit; each section of the system can maintain its own pressure even if an adjacent section fails. SYSTEM PROTECTION VALVE: An extension of the check valve concept — the single system protection valve on the supply tank prevents the primary and secondary tanks from bleeding back to the supply tank in the event of supply tank failure. TRAILER APPLICATIONS: In tractor-trailer combinations, check valves also prevent trailer brake air from back-feeding into the tractor supply when the trailer is disconnected while pressurized. PRE-TRIP INSPECTION RELEVANCE: While drivers typically don't test individual check valves during pre-trip, the check valve function is verified indirectly through the dual-circuit leak test — if pressure in one circuit drops while the other holds, it indicates the circuits are properly isolated, confirming check valves are functional.