[Oldsguy]

Also, you might have an actual ridge built up at the edge of the drum which is preventing it's removal, if the previous owner never maintained the brakes then you might have to use some very agressive "encouragement" with the mallet, you probable won't damage anything in the brakes by hitting too hard except perhaps a retainer spring and if one of those go then they were too old/rusted and ready to be replaced anyway. C.J. didn't mean hitting the backing plate, hit the drum itself, first on one side and then the other. What you are trying to do is knock the crud and rust from the brakes and get them to recede (sp?) back to their resting position. If you don't know what the brakes look like or have never seen them or can't visualize it then you really should find a maintenance book that has a good diagram of GM drum brakes. It will help you immensely. Also, something like PB Blaster might help if your car has the little adjusting slot on the backing plate, you can put the straw up into that slot and spray the daylights out the interior of that drum, it might help break all that rust and gunk free. You will have a mess to clean up but brakes are really messy anyway and you may end up having the drums turned or replaced. If you plan on doing the brake job yourself, include some brake cleaning spray in your consumable supplies to make the job easier.

Just thought about something else, it could be that the wheel cylinder has some scoring on the inside of it that is preventing the brakes from returning to the resting position, when you do get the drum off, inspect that cylinder closely, as you said your are troubleshooting a leak anyway, it might be better to just replace that cylinder. It is usually cheaper to do that than hone and rebuild them anymore and some people always do them in pairs, especially the fronts but it's up to you.