Morris Minor Owners Club

had a problem adjusting the rear O/S brake - plenty of meat on shoes, but wouldn't lock on the adjuster. replaces drum, everything OK. Same problem on front O/S, the rearward adjuster doesn't lock. (not quite - it gets "stiff!). The forwards adjuster locks. Again plenty of meat on shoes, new drum on, makes no difference to the rear adjuster.
Snail worn? Anything else it might be? Is it easy to replace the snail?

Yes, very easy. It might make sense to replace both the cam and the mask because they wear against each other.

I would take the drum off again, and check its inside diameter is exactly the same as the old drum. by the way why are you fitting new drums? I would also check that the shoes are mounted properly on their supports. turn the snail with the drum off, see if it is actually moving the shoes up and down.