If the capacitor passes these tests, it only indicates that it might
be alright in normal use. The condenser will get hot when in normal use so we put it in the oven for an hour at about 60-70°C and test again. This is because the insulation properties do deteriorate as the temperature goes up.
The mica condensers, although up to a hundred years old, more often than not test out OK. If they need to be replaced, it is usually because of physical damage rather than a loss of electrical performance.
As for the paper condensers, well to be honest we used to get so many failures when testing that we don’t even bother now – we always replace them with new ones. Even NOS (new old stock) ones fail.
CAUTION: Sometimes vintage test equipment becomes available but be careful. We have a combined coil and condenser tester which, when we used it to 'test' a condenser, it passed even though we had already rejected it. Further examination showed that the test meter only used 60v to test the insulation........ The tester makes a very nice looking ornament and a good conversation piece but it's not much use for anything else!