What was found
(CAN) FLIGHT CREW NOTED AIRCRAFT HAD DIFFICULTY REACHING MAX DIFF 4 PRESSURIZATION WAS FLUCTUATING. DURING TROUBLESHOOTING IT WAS NOTICED THAT A BLEED AIR PIPE IN LH WING HAD A LARGE CRACK AROUND THE WELDED - 4 FITTING ALLOWING BLEED AIR TO ESCAPE INTO WING. EVA TUBING DID NOT RUPTURE AS THE LEAK WAS OPPOSITE OF TUBING AND DID NOT GIVE INDICATION OF PIPE FAILURE. AS WELL A FAULTY DIODE IN WIRING TO LH BLEED SHUTOFF VALVE PREVENTED VALVE TO OPEN WHEN LH BLEED SELECTED. LH BLEED SYSTEM WAS NON-FUNCTIONAL. BLEED AIR PIPE AND DIODE WERE REPLACED. LH BLEED SYSTEM AND PRESSURIZATION WERE GROUND TESTED WITH NO FAULTS NOTED.