Oil Gauge Repair??

[bmc164]

I recently bought a 1983 240d. Among its numerous minor problems was a leaking oil gauge. I took it to a local MB shop where they replaced the gauge with a used unit (for which they charged me $175). Now, less than a month later, that one is leaking worse than the one they replaced. I'm sure as hell not going to pay $400 for a new gauge cluster, but I'm wondering if there is any way to a) repair/ rebuild the gauge itself, or at least b) test a used guage to make sure that it's not going to leak in a month. Any feedback would be greatly appreciated...