On some engines the valve tip can fall below the valve guide making it difficult to pull back up. On some engines the valve will just rest on top of the piston.

Air is used when using a lever valve spring compressor. A twist type compressor ( look up K-D tools ) can be used without air however it becomes difficult to remove the keepers as the valve is trying to fall into the guide.

Look around for one of those 3 gallon carry compressors on junk day / pawn shop / flea market. It will have enough air to do the job if the valves are sealing reasonably well and you don't take too long. If you get into a situation where you must disconnect the air, wrap a rubber band around the valve stem ( at the keeper area ) to keep it from falling in.