There should always be some resistance when the piston gets close to the top of its stroke. As it passes the top of its stroke, you should hear/feel the pop of the compression releasing. If the engine is not currently overheated, is really difficult to tune (and overheats really frequently - but has no air leaks) and you have no resistance (But the piston is going up and down while you pull), it is probably time for a rebuild (new piston and sleeve).