When I get one of these old machines, one of the first tasks I do is I take down the water level and see what the condition is. Usually the pressure plate for the seals is affected by rust. The plate is then meticulously cleaned of dirt, paint residue and rust. Then I paint it 2 - 3 times with rust converter. Works great as a primer and prevents rust from forming again. Then I paint the plate with a commercially available enamel paint from the model building sector. I have not yet tried the Wilesco cardboard seals. These gaskets are very good for sealing connections from steam whistles, filler plugs and safety valves to the boiler. However, they must be replaced depending on the frequency of operation. I therefore prefer the homemade silicone gaskets, which are much more flexible. Important when installing the water level tube and the pressure plate is the absolute cleanliness of the parts. Also the holes in the boiler for holding the tube, and their surroundings must be metallically bright, so that the seals in the surface can lie cleanly.
On a machine that was visually in good condition, I have experienced a nasty surprise. I have not paid the necessary attention to the water level and heated the boiler. The machine started very well. After about 1 minute steam came out at the seals of the tube. The discharge was then explosive and caused a huge mess. One of the previous owners had sealed the water level with window putty. Water, heat, window putty and pressure, not a good clash.
Fortunately, no one was hurt. That was a lesson to me though. You can see why I'm a little picky about this now.