I'd suspect the seal on the pump basket lid. Reason I say this is on high speed the suction is a lot more than on low and pulls the lid for a tighter fit and better seal. All it would take is a small piece of dirt. Clean the gasket and use ample silicone lube and see if that helps. Also, on low speed there may not be enough flow to completely purge the air bubble in the basket. I wouldn't worry about that.
Al
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