Excess Letdown established to VCT via CV134 (3-way valve). Key valve identifications:
- 2CV278, 2CV131: Excess Letdown Isolation Valves — do NOT receive automatic closure signals on SI/Phase A
- 2CV284, 2CV116: Seal Return Isolation Valves — close automatically on Phase A Isolation
- CV115: Seal Return Relief Valve — relieves to PRT (not RCDT)
- CV134: 3-way valve directing excess letdown to VCT or RCDT; fails to VCT on loss of power and air

Following an SI signal, seal return isolation valves CV116 & CV284 close via Phase A, blocking the normal seal return path. Excess letdown flow continues through CV278/CV131 (which remain open) but is trapped downstream, causing CV115 to cycle and relieve flow to the PRT. Trap: excess letdown isolation valves (CV278/CV131) do NOT close on Phase A — only the seal return isolation valves (CV116/CV284) do. Eventually, control air bleeddown may fail CV278/CV131 closed, but this takes considerable time.