There is an internal spring that makes the plunger top out against the snap ring when the load of the pushrod is released by disassembling the valvetrain. If the plunger is stuck in the down position there's a problem. Lifters can be disassembled and cleaned with something like spray carb cleaner, but do it one at a time to avoid mixing up parts. There are some incredibly tight fits inside there, and mismatched parts usually don't interchange. Watch how things come apart if you choose to open one up, and memorize the relationship of the pieces or draw a sketch for reference.
Jerry
