Not sure if you can do any modification like this without some risk or a lot of work. There are some other posts on gas ballasting and you can do a lot in terms of technique that minimize the need for a gas ballast in straightforward applications like the fusor.
FYI I am horrible in terms of attention to this sort of thing. I do change my oil when it starts to get ugly looking but I don't sweat low base pressures in the mechanically pumped side of my system. If I need lower than 30 or 40 mT I use a high vacuum pump simply because if you need pressures lower than this, you probably should go at least one to two orders of magnitude lower in terms of base pressure.
Where I am scrupulous is with my freeze drying rig where I have a dedicated Kinney rotary piston pump with an extremely effective gas ballast and a molecular sieve trap. The water load from the freeze drying sample is too large to ignore.
Steve
Created on Sunday, April 01, 2001 10:35 AM EDT by Steve Hansen