The age of your 5th may say you need a new gate valve, GATE VALVES
, you will need to dump tank and remove the gate valve blade. If the slide chamber is ok than you will just have to replace the blade, rubber rings and four bolts.
If not you may have to do some pipe work also.
The long rod will turn out of exsiting blade to rescrew into new replacement blade.
Your four old bolts maybe rusty so be careful removing them to save the blade holder sides.
Once you have apart replace the old rings slip in blade and replace four bolts. Only tighten bolts to where there are no leaks and not bind the blade so it can not move but moves freely.
The valve if enclosed must have a panel to remove to gain access maybe someone with a 5th can help you with that.