I installed a new set of Robart pneumatic retracts on my P-51 and after a few flights, one of the retracts is sticking. That is, rather than fold up and down easily, it requires quite a bit of pressure--enough pressure that it can no longer be operated by air.

I've taken the retract apart. It doesn't appear that the mounting brackets are interfering with the retract mechanism, and as far as I can tell, the shaft going into the air cylinder isn't bent, either. Has anyone experienced anything like this before? Is there some kind of lubricant I should be putting in the air cylinder or anything?

Sounds like you should send it back for repair or rather replacement. Robart is rather good about product support. There may be a problem internally. Rare, but it can happen.
No lubricant is required normally.
[&:]

I like to put a little air tool oil in mine every once in a while. Yours being new though sounds like another problem. Don

I had a problem with Robarts binding that I traced to the four pieces of threaded barstock that space and support the side frames. They were not all the same length. Micrometer or calipers required to measure the difference. I had dissassembled and did not get them back in exactly the same position. This caused the cross bar on the end of the cylinder rod to bind up in the side frames. I don't remember if I shimmed them or just matched them for reassembly, but They worked fine after that.

Mine has over 60 flights on it and the same problem has cropped up twice after a little excessive sideloading of the gear in a crosswind. What usually happens is they retract but will not extend. Here is the fix, but be careful not to break the gear mounts...go slow. To extend the gear when it does this push up on the gear to unlock it. Once its extended you need to "tweak" the gear outward toward the wing tip, just slightly. Yes it takes some force but nothing excessive. The gear will then work fine until the next time you side load them too much.

Interesting. I'm almost certain side loading caused the problem because on my last landing, I zig-zagged the plane pretty hard after touchdown to keep from running off the end of the runway. I have the retract disassembled now and am searching for the source of the binding. Thanks for the input!