Small Engines (Lawn Mowers, etc.): Fuel problem, craftsman lawn tractor, crank case

Question
Hi Robert,
I have a 1976 Craftsman Lawn tractor with a 12hp Tecumseh 4 stoke engine. Im having a problem in getting fuel to the carb. When trying to start the tractor after it has sat unused (usually for 24hrs or more, sometimes less)it loses the ability to pump fuel.It will start but once it uses the fuel in the float bowl it will quit. If I preasurize the system it will start.(I preassurize the system by blowing into the gas tank by mouth)It sometimes will not have any problems after I do this but if it is left at idle it sometimes quits again. Once started if I go to a fast idle or full thottle it will stay running and I have no problem until I try to start again sometime later . It acts as though it loses it prime. I have replaced the fuel line,fuel filter(a number of times), rebuilt the pump,replaced the hose from the pump to the crank case and cleaned the carb. I have checked to assure the float isnt sticking and have no problem with fuel flow to the pump. This engine has 110lbs of compression,burns no oil and runs very well. The oil is changed regularly and is kept at the correct level. The gas cap is not plugged(I have had the same symptoms with it off). Could it be low crankcase vacuum that isnt allowing the pump to work correctly? What is normal vacuum? Im at a loss. I have asked local repair shops for thier help but most dont feel its worth working on because of its age. They are more interested in selling me a different tractor. Any ideas would greatly be appriciated as Im tired of blowing in the gas tank and I cant convince my wife to do it.(ha ha)Seriously it is a good tractor and in great shape other than this annoying problem.
Thank you, Dennis  

Answer
Dennis, great information !! but a short response...First check the vaccuum lines to the fuel pump. Does the fuel pump mount outside? I'm presumeing there is a external fuel pump. If so.....the pump may have to be taken apart and the diaphragms checked. One or more may be deterioted or the "flappers" are degenerated.If you can't get parts, I would suggest thata fuel pump "external" from a 16HP cast iron engine will work. It's a matter of Vacuum In, fuel line In and fuel line Out.... Let me know qwhat happens...Bob