You will need to remove the cover on the tower to get to the linkage. I had a similar issue when I first got my FLU419 only it impacted both swing pedals. In my case I had two problems. Problem #1 was the valve assembly had some surface corrosion on the valve rod that slides up and down. It would only go up and down a small amount. I cleaned it up with a wire brush on my drill. The second problem was the pins in the linkage were rusty. Actually, all of the pins for all of the controls were rusty. I removed the entire linkage, tower, etc. cleaned everything up and painted the parts, greased everything and put it back together. That was years ago and no problems with the swing pedals since. One mistake I made was not taking photos of the linkages before I removed all of them and mixed them up. It was like solving a jigsaw puzzle when I reassembled.
As a side note, I am now having similar problems with the left stabilizer. I located an NOS valve assembly on Ebay for $100.00 and it arrived today. It looks new. I'm in the process of dismounting the backhoe to replace the valve assembly for the stabilizers.