Dispatched with a deferred right engine PRSOV (Pressure Shut Off Valve); a B757-200 diverted to a close airport when the operative pressurization system associated with the left PRSOV malfunctioned and a large area of thunderstorms loomed enroute to their destination.

Narrative

The right engine PRSOV (Pressure Shutoff Valve) was deferred inoperative/closed and the left engine bleed was driving the left pack (RH pack off per MEL). In cruise at FL 350 the Captain felt a pressure change in his ears and asked if I felt the same. We both looked at the pressurization panel and noted that the cabin altitude was climbing at 500 feet per minute and the cabin differential pressure was decreasing. After discussing the situation we opened the isolation valve; turned on the right pack and turned off the left pack. The cabin altitude decreased to normal after having reached approximately 8000 feet. We contacted Dispatch and Maintenance Control and explained the situation. All parties agreed a divert was the best option. At the time; there was a large line of thunderstorms in the Valley area and to continue to the coast with possible loss of the remaining pack and need to descend would put us in the weather. We made an uneventful divert. Within 15 minutes of block-in; maintenance discovered the cause of our problem; a hose clamp for the left pack ducting had come loose. We were given another aircraft and continued.