I think the "down springs" in my Robart air up spring down gear took a set after nine years and would not lock down the gear. I got the HK heavy duty electric retracts in some time ago, but finally installed in the Macchi.
http://www.hobbyking.com/hobbyking/s..._10_12kg_.html
My Robart struts were slightly too large to fit into the struts so I had to drill out the struts some. Enough metal so not a problem. So far six flights on them. They work great.