Over time, the probe and the piston in the Duratrac 4 Streaming Current Sensor will become worn, and subsequently less responsive. When this happens, cleaning the probe and piston surfaces may not be enough to improve the response to a degree of satisfaction.
Typically the piston will be changed once, or even twice, before the probe sleeve needs to be changed, and that will restore an adequate response. But when changing the piston in not enough, the sleeve should be changed as well. This video shows the procedure for replacing that sleeve.