Coley might be on to something with the impeller.

Isn't there a separate impeller for cooling/pissing? You said it worked at the house when hooked up to the garden hose but keep in mind the garden hose is "helping" by it's own pressure forcing water into your intakes.....may have offset what a weak cooling impeller couldn't handle anymore. I'm going on the assumption you were using one of those earphone-looking contraptions you hook up to a motor so a garden hose can be attached.

When you had it broken down did you remove any shafts/gearing that might control the cooling impeller?