Daewoo/Solar 450-III boom hydraulic issue

I have an issue on my 450 Daewoo, for some reason that I can't find, the book will hydraulically lock when trying to go down. Sometimes you can get it to work by moving the stick, but other times you have to take what appears to be a check valve out to get it to drop.
Hope someone has some info for me on this. Thanks

Daewoo/Solar 450-III boom hydraulic issue