The drum has probably split at the snap ring groove. This part is at the last time of checking, not available. To be sure it is that, lower nearside subframe and gearbox mount to gain access to the pump housing. Squirt plenty of penetrating oil around edge as ally corrodes. use suitable slide hammer to remove pump. Try and keep it square and maybe working it back in and out. Taking apart is the easy bit. The suspect part is in the first part removed after pump.
mags
March 2011