realizing that the treatment of leaking magnets obviously
is a hot point of discussion among ammrlers, I want to contribute
our experience with an old oxford dewar (250 MHz).
The magnet went on field in December 1979 in a medium sized lab,
and initially no care was taken to vent the helium boiloff to the outside.
Towards the end of the eighties (after approximately eight years of
smooth performace), the boiloff rate started to increase (measured
by the dipping method once a weak; no He meter available then).
When the initial boiloff had doubled, we asked our low-temperature
physicists for help. They mounted an oil diffusion pump directly
to the outlet of the magnet, using a turbo pump somewhat distant
for generation of the pre-vacuum (no problems with eddy currents;
the fact that both pumps will keep working for a short period after a
power failure enables you to replace a safety valve by a cheaper device,
e.g. by a student keeping watch and closing the magnet valve when the
lights go down; overnight, the magnet valve was closed, put the pumps
were left running).
Pumping for about a week restored the boiloff rate to its initial value.
We had to repeat this procedure three to four times (I don't remember
exactly) in decreasing time intervals (from 1.5 years at the beginning to
ca. .75 years at the last time).
In 1992, we finally had the magnet serviced and the seals replaced.
Afterwards, it exceeded the specifications from 1979 (concerning the
boiloff) by far and still does.
To make a long story short: leakage due to aging of seals (being chronical,
not acute) may well be kept under control for a time by cold pumping -
at least until funds for a general restauration are available.
Hope this is of interest to some of you; regards, Hermann