  Recent suggestions that buoyant radio emitting cavities in the intracluster
medium can cause significant reheating of cooling flows are re-examined when
the effects of the intracluster magnetic field are included. Expansion of the
cavity creates a tangential magnetic field in the ICM around the radio source,
and this field can suppress instabilities that mix the ICM and the radio
source. The onset of instability can be delayed for ~100 million years, and
calculation of the actual reheating time shows that this may not occur until
about 1Gy after creation of the cavity. These results may explain why the relic
radio bubbles are still intact at such late times, and it may imply that the
role of radio sources in reheating the ICM should be re-examined. In addition,
the existence of relic radio cavities may also imply that the particle content
of radio source lobes is primarily electrons and protons rather than electrons
and positrons.
