I'm working on a Doosan D30S-7 forklift. About a month ago it was having issues with the brakes applying while in use. Everytime I would show up it wouldn't happen. Finally I saw it. It started doing it all the time. Leave it set for awhile and go again. It has hydraulic assisted brakes. The brakes are wet type. When the issue would happen I could crack the blades and release the brakes. I finally came to the conclusion that hydraulic pressure on the assist side of the master cylinder was bypassing applying pressure to the brakes. I installed a new master cylinder and it worked perfect. I got a call yesterday saying the condition was back. I drove out and tested the unit for about 30 minutes with no symptoms. I'm sure they will get more frequent like before. I cant imagine the new master cylinder is bad, but it's possible. Is there something I'm missing. My contact at Doosan has never heard of this issue. Thanks for any help.