Hey Kev, It could be several causes but I follow this process: Remove the drum, clean the brake assembly and drum with brake cleaner to degrease. Measure drum thickness and shoe thickness with a micrometer or caliper to make sure minimum thickness at least. If not then replace them. Then when fully dry rough up the shoes just a little with 80 grit paper. If there's a big lip on the edge of the drum remove it with a die grinder and burr or similar if possible and rough up the drums a little in a circular movement with the 80 grit, just a little you are not trying to remove any scoring. Then degrease the drums and shoes again with brake cleaner. Remove the brake adjusters, clean and put a smear of copperslip on the threads and barrel as you reassemble. Put a small smear of copperslip on the balance bar and pins (rear only) and on the brake backplate at the raised parts where the shoes come into contact with it. Do no get any on the linings. Try to get the shoe retaining springs to sit evenly and ensure the shoes have an even gap all the way round before putting the drum back on.Then reassemble and adjust as normal. Readjust after 100 or so miles if shoes are new.
This is roughly how I service drum brakes and should ensure that everything is as it should be. If this doesn't work and nothing is broken then it's possibly the shoes, how cheap were they and what brand?
Just a thought but if the squeal is there all the time it may be part of the lip on the backplate touching the edge of the drum where it sits in the groove on the drum. Turn the wheel round slowly and you should hear it scrape. The backplate is easily bent if it's old and rusty so check while the drums are still on looking from behind and then check that the plate is not bent anywhere after removing the drums. This has happened to me before and is easily missed.
Cheers
Dave
