Paul,
It can be cleaned, the master cylinder is at the bottom of the page you have posted. It's where the #2 assembly, a seal rebuild kit goes into. The #4 oring that goes on to the unnumbered elbow is where you want to look at. You'll see a tiny hole inside the body of the master cylinder and that's your return orifice hole. I usually clean them with a single wire on a wire brush. I would suggest that you purchase the seal kit, #2 in the picture and rebuild it while you have it apart. If you have any questions my number is (936) 232-9299.