Cmac- If the pump works fine without gfci protection, but trips the gfci even after replacing the receptacle you have a ground fault in the pump, or the cord going to the pump. It may not be enough to trip the circuit breaker, but can still pose a hazard to human life. The first thing I would rule out is where you spliced the cord,(not something I would have done for a customer!) even if it was never underwater moisture inside can cause a gfi to trip. If thats not the problem it's the pump. A sump pump has GFCI protection for a very good reason- the whole mixing electricity with water thing- so don't bypass the protection and try to claim compliance with a long ago superceded code. Your customer and your conscience will appreciate it. Do what has to be done to make it safe and code compliant, and if they don't want to "pony up" let them find someone else willing to risk their reputation so they can save money. good luck!

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1984 300TD 1981 240D