Definately pull the prop valve apart. If it hasn't been sitting around unused for a long time, it will probalby come apart easily. I just had mine apart a few months ago. Gary(Challenger_1) posted a great picture of one disassembled. I saved his pic but don't have it on this computer.

Soaking it in EvapoRust should clean it up nicely. Just clean it out the best you can before you dip it so you don't dirty up your EvapoRust too bad.

Once I had mine reassembled, I applied RPM to it so it won't rust again.