PA-702 Captain reported the left bleed valve light was not illuminated and while attempting to fix it; inadvertently depressed the switch; which then got stuck in the closed position; and the aircraft started to slowly depressurize. Reporter was able to re-open the valve and the pressurization system returned to operating normally.

Narrative

Leveled at FL230 and completed cruise checklist. Scanned instruments which included looking at pressurization system (situated on center pedestal). Noticed the Left Bleed Valve light was not illuminated. Valve had to be opened as we wouldn't maintain pressurization with just one bleed at this altitude. Tapped bleed valve switch as sometimes the light is a loose connection and light didn't illuminate. Tapped harder and inadvertently depressed the switch which closed the left bleed valve and cabin started to slowly depressurize (cabin was about 7;000 feet). Pushed the switch again and it would not open (stuck in the closed position). Hit it again and it wouldn't open; cabin was now approaching 10;000 feet so I started down and requested lower as the cabin was going to quickly exceed 12;000 feet. ATC provided priority handling. As I started down; hit the switch again as the descent was established. By then ATC had cleared us to the lowest altitude (16;000 feet). At about 19;000 feet I was able to open the valve and the cabin started pressurizing; but continued the descent to 17;000 (coordinated with ATC) to consider options. Ultimately; I requested higher once I determined the pressurization system was operating normally. I may include cycling the bleed switch periodically to assure free movement as this switch is normally not activated. Maintenance looking at switch.Time from event to level at 17;000 feet was 4 mins 41 sec. (From FOQA data.) Highest rate of descent was 2;850 fpm at approximately 20;640 feet.