I thought I would report on a successful repair of two valve actuators that stopped working on me. Rather than spend a few hundred bucks replacing them I poked around inside and found that the limit switches that start and stop the valves were faulty. A quick search on ebay led to replacement parts for $16 per valve. After a 5 minute repair job the valves work like new.
Look for item number 221537493021 on ebay.