Hi there.

It looked to me like the only difference between the Guzzler 400 and 500 was the size of the inlet/outlet: 1" for the 400, and 1.5" for the 500. The pump bodies are the same size, about 5" diameter. I'm going with the 400 just because the smaller tubing required will be easier to install.

In the pcarter site mentioned earlier, using a bungee for the return stroke, i.e., the suction stroke, seems easier to figure out than getting a properly configured metal spring to mount internally. It only has to be strong enough to lift water up into the pump body, not very much in a kayak, a couple of inches. My starting point will be trying 12" of 1/2" bungee; I think this will be close enough. A point to keep in mind may be that any "excess" force the spring or bungee generates will have to be made up by you with every stroke, so the biggest or strongest spring available may be more than what you want. I'm thinking the bungee thing can be made adjustable to find the best balance between foot effort required and speed of refill on the suction stroke.

The Guzzler pumps are small enough to fit the forward bulkhead. For the conventional footbraces this won't be a problem, as it could go right between them. For braces in the form of a large panel, as in Nick's book, I don't know how you'd do it, but there's got to be a way.