We all experience lot of problems pressurising 737 classic aircraft only with apu, specially old aircrafts with ETC. Here is the solution: ETC limits EGT by closing apu bleed valve which in turn is not able to provide enough pressure to pressurise old leaking fuselage. ETC EGT limit margin can be increased by simulating main engine start mode (MES MODE) remove any start valve connector and move and hold start switch to GRD position. NOTE USE MES MODE ONLY FOR LIMITED PERIOD ENOUGH TO HELP AUTO FLOW VALVE TO CLOSE. MONITOR APU EGT CLOSELY NOT TO EXCEED MAX CONT EGT ! DON’T FORGET TO PUT START VALVE CONNECTOR BACK !