You're only pumping for about a quarter of the wheel's rotation. (As I read it only part of a half-turn, since the piston only seems to be pressurizing in one direction. That gives it a half-sine-wave load.)
You could add a stepped crank and a couple more pistons to get "three phase" out of it and load it for a full rotation.
Alternatively you could add an off-center weight (or a weight on the piston's rod). On one half-turn the wheel would raise the weight, on the other the wheel and the weight would push the piston. Then you'd load the with two sine waves, i.e. over a full half of its rotation.
Adding enough mass would let you store energy as momentum for the 3/4-equivalent of the rotation where the wheel is otherwise not being loaded.
