Pool Pump Keeps Tripping Main GFCI Breaker after 2-10 minutes

[dabaxter]

I have a 40A GFCI Breaker feeding a pool filter circulation pump, a polaris booster pump, and a pool fountain feature pump. Each is fed by it's own breaker downstream of the main 40A GFCI Breaker. The pool fountain feature pump is a whisperflo pump. It has a 3/4hp motor. It has run trouble free for years. Last spring the main 40A GFCI breaker began tripping. I replaced the breaker. This spring when I went to start using the pool fountain feature, the motor was failed and heaviliy corroded. I recently replaced the motor. When I turn on that pump, it runs for 2-10 minutes and trips out the main 40A GFCI breaker, but not the downstream individual breaker feeding it.
I can't figure it out. Any suggestions?
Thanks!