Similar to Andrew, The slide valve on my Ruston TE has a slide valve on the side which kept falling off and not sealing when steam was shut off. Then bang back on when turned on.
A simple flat leaf string attached to the back of the slide valve (made from "Band-it" steel strapping) and rubs against the back of the cover plate fixed the problem. Very little force is required to hold the valve, just enough to keep it seated.

Edited By Paul Lousick on 03/10/2023 22:19:05