A Mechanic reports that he heard a loud noise when he turned on the the 'B' System Hydraulic pressure on a B737-300 aircraft to check brake wear on the right main gear; during a dayshift C-Check overtime. The leading edge Krueger flaps had extended; damaging the opened #2 Engine inboard thrust reverser sleeve and hold-open rod.

Narrative

While working dayshift C-Check overtime; I was assigned wheel wells/struts. One of the non-routines [(write-ups)] was that the #3 brake was worn. After viewing the indication pin on the brake; there was obviously well over the allowed pin length remaining before a brake change would be needed. The only way to accurately measure this length is to turn hydraulics on and set the brakes. After clearing (notifying) the mechanics working on the trailing edge right wing; I cleared the mechanics working on the tail. Then walked around the left wing where no one was working and after yelling 'HYDRAULICS;' proceeded up to the flight deck.I checked the flap handle position to ensure it was at 40-units; and turned flight controls off. I removed the guard on the hydraulic switches; which I assumed were locked out from the mechanics working on the trailing edge wings and turned on the 'B' System Hydraulic power. I immediately heard a loud noise and turned off hydraulic power. The leading edge Krueger flaps had been retracted and the #2 Engine inboard thrust reverser was open and when hydraulic power was applied; the Krueger flaps extended which caught the thrust reverser sleeve causing damage to the sleeve and the hold open rod. I realized that I had cleared the mechanics working the wings; but had not looked at the position of the leading edge and #2 inboard thrust reverser.