Replacing the O rings should have cured any contamination problems,
as those rings were never in the ultrasonic.
The only thing I can think of is that Goss brand presto-lite style
torches have a different gasket setup than the normal Smith or
Presto- Lite versions.
Smith and P-L use O rings on the sides of the tip shafts, while Goss
has a flat rubber washer down at the bottom of the handpiece socket.
The idea is that the tips seal down against the washer. Those get
torn up from time to time, but they’re replaceable too. Check to see
if your handpieces are Goss brand. They all look pretty much the
same, and they’ll take the same tips, so you wouldn’t notice unless
Otherwise, inspect the seal shafts inside the handpieces, to make
sure there’s no gunk in there that’s fouling up the seal, and try
replacing the O rings again. Go to the gas house, and get acetylene
rated O rings. It’ll chew up normal hardware store O rings.