The cause is bad seals, you can get new seals and fix the leak. You will need to un-do the linkage at the front, clean it up well, remove the covers front and back and slide the spool out the back of the control block.
There are detent balls in the back cover, be careful when you remove it, they will go flying.
When installing the new seals, make sure they go the correct way around, take note of the way they came out.