Most tubes are simply an interference fit in the block, and will just pull out if you offer enough force. But you could mangle it in the process if your not careful. If you can get under the lip with a flat tool like a thin scraper, you might be able to work up out of the hole. They can really suck to remove if they've been in a while. Often the parts stores have a generic tube that could be altered to match the original. Just remember to measure the exact height of the top of the tube, so whatever you put back matches, or your readings will be off.