The other thing to note on these type of locks is that the little springs that live inside the lock barrel can become gummed up as the grease inside hardens up and gums up with little fragments of the brass keyways over time.
Its easy enough to do, remove the handle and lock assy, put the key in the chamber, as normal, and then remove the bits at the back of the barrel so that the barrel itself, complete with key still inside, can come out. Keep your fingers on the little brass keyways and DONT pull the key out - otherwise you risk the lot pinging out everywhere and then the fun starts trying to work out where the respective keyways go
Run a little 3-1 oil onto the tops of the key ways and gently begin, while holding everything still, as above, to ease the key in and out and then gently let your fingers ease pressure to see if the keyways can move or not. if they dont then the lock cant do its job so you need to keep dripping some oil in until they move up and down as you put the key in and out.
Basically the keyways go down with the key in, and out - and into the lock chamber so as to hold it in the lock position - Once thats workings fine, reasemble and try it for size, you should be ok now.
Doug.