Lucille Same here.I took mine apart and cleaned with Brake Clean, then let dry and coat with brake assembly lube or brake fluid, Another thing you should look at is the thread where the adjuster bolt enters the end cap.The threads can be loose and are sometimes a poor fit. Use you're old end caps if possible. Also inspect the seal cups for very small metal flakes Gently remove by rubbing with finger nail or tooth pic. A another area to look at is the adjustment spring that is riveted or screwed on to the cylinder and keeps the adjustment buy resting in the notches on the end cap. If it is not centered or loose it will need to be corrected by replacing or screwing it on with another from old cylinder. If not centered the end cap can move out from under it and not let brake release or stay in adjustment if it is loose.
I too just got two cylinders from NAPA and needed to clean them. But they did have screwed on adjuster springs that where well centered. but the threads in the end caps where a poor fit.
If I had a lathe I would make a set with good threads and an adjuster notch that one could actually see threw the slot in the backing plate. More like the cast iron one's on the rear of my one ton.