Hotpoint HG2 Grill Thermostat bulb?

This is a Hotpoint Grill from about the late 1940s. NO parts are available so I HAVE to fix the existing parts.

Tha capillary tube got worn through and the liquid escaped. I cut the bad part out, filled the bulb with alcohol and sleeved the tube. The thermostat now goes on and off but the alcohol is TOO sensitive and the unit does not go above 200 degrees.

What I need to know is WHAT "thermally sensitive" liquid do I need to add to the bulb the handle a temperature of 400 degrees?

I was thinking something like baby or lamp oil might work. Maybe just plain old liquid Crisco! I need a liquid that will not vaporize under 500 degrees.

Thanks,
Dwight Lanning
dlanning@bedford.k12.va.us