A MD80 Crew reported diverting after the left engine pneumatic system was found with uncontrollable pressure in excess of 110 PSI. The Crew subsequently refused the maintenance department's request to MEL the valve.

Narrative

During climb out I noticed the pneumatic pressure gauge reading 70 PSI with both pneumatic crossfeed valves in the closed position. I turned on the airfoil anti-ice in accordance with the operating manual to see if the pneumatic pressure would drop to zero. With the airfoil anti-ice on and both pneumatic crossfeed valve levers in the closed position; the pneumatic pressure gauge continued to read 70 PSI. This was highly unusual. After the aircraft was leveled off at FL250 (due to a single pack restriction) and cruise power was set; the pneumatic duct pressure read 40 PSI with both pneumatic crossfeed levers in the closed position. I called Dispatch and got a phone patch to a Technician on the cockpit air cell phone. I described the highly unusual situation of the pneumatic pressure not reducing its indication when the airfoil anti-ice selector was placed in the 'on' position and both pneumatic crossfeed valve levers were in the 'off' position. The Technician requested that I open up one crossfeed lever at a time to identify which pneumatic system (left or right) was causing the problem. I opened the left crossfeed valve and immediately the pneumatic pressure gauge pegged at over 110 PSI. I immediately closed the left crossfeed lever and the pneumatic pressure gauge returned to 40 PSI with both pneumatic crossfeed levers closed. It was obvious from this high over pressure that the left engine high pressure bleed valve was stuck in the full open position and I had no way to stop the over pressurization of the left pneumatic system or to stop pressurizing the cross bleed manifold even with the pneumatic crossfeed valves fully closed. Additionally; I was not sure of the pneumatic pressure the left engine high pressure bleed valve was putting out during cruise flight because the pressure gauge had pegged out at 110 PSI when the left crossfeed valve had been opened. Because I was not able to control the over pressurized left pneumatic system; I had no way to stop pressurizing the center crossfeed manifold; I decided to divert in the interest of safety for my passengers and crew.