drum brake pistons

Hi all i just spent the last two weeks towing the camper trailer around the south island. Found i was having trouble pulling it up and getting alot of brake dust around the front wheels not far from home the hand brake light came on which means its low on brake fluid had a look round and there was fluid around the left back wheel (blown piston seal) brought 4 new seals only $10nz for both sides. When i pulled it apart i found the piston that opens the rear brake shoe was sized closed and had not been working for some time i checked the right hand side and the same rear piston was sized. has anyone else had this happen. it doesn't take long to check them and a little bit of grease under the dust cap will stop this happening.