Air carrier technician reported during an operational check of the aircraft brakes they found the hydraulic system reservoir charging shrader valve opened.

Narrative

NOTE: THIS REPORT IS BEING REPORTED DUE TO AN IDENTIFIED DEFECT THAT WAS CAUSED BY PREVIOUS MAINTENANCE. I was completing XXXXX for re-tensioning both brake cables. Upon completion I decided to check brake pedal feel and travel to ensure no binding or any other defects (not required by amm (Aircraft Maintenance manual) ). Both Capt and F/O side pedals felt fine without hydraulic pressure applied. Once pressure was applied; a loud horn like sound was heard coming from the R/H side of the A/C; and a vibration was felt in the pedals. With my partner cycling the brakes with the system pressurized; I located the sound coming from the R/H wheel well. I originally assumed it was the R/H brake metering valve. I then talked with my partner and thought the Hydraulic Reservoir pressure could be low. Upon checking the gauge in the wheel well; I found the Reservoir pressure gauge reading zero with the charging shrader valve to be opened; and dust cap missing. Logbook Deferral Sheet XXXXXXX was then created and closed by charging the system with N2 and replacing the dust cap. The system was then re-pressurized to cycle the brakes which then corrected the issue. The hydraulic pumps were monitored for overtempt or any other abnormal conditions which never occurred and the A/C was returned to service. I then talked with management to discuss a course of action to report the issue; and they directed me to call Maintenance. Maintenance then said that there is no internal reporting for this kind of scenario and an report should be filed. The A/C blocked in at XXXX and arrived to the hanger XXXY (not sure if it was a taxi or tow to the hanger). The scheduled overnight maintenance did not have any items requiring the hydraulic reservoirs to be depressurized; or any items even relating to hydraulics. While I was working; my partner was completing a transit check; and confirmed that he did not touch the valve. Again; this report is to identify an issue caused by a previous maintenance action. Neither myself or my partner opened the schrader valve.