A unit tripped during cooldown due to low hydraulic pressure and failed THLO pump engagement, causing a four-hour lockout. SISO recommended accumulator and pump checks, along with offline pump testing and trending. Analysis revealed hydraulic pressure dips when variable geometry components returned to offline positions, triggering pump failure. By adjusting pump flow and observing VBV pressures manually, SISO stabilized pressure, resolved alarms, and eliminated related circuit failures.