I cleaned up a lightly used ECHO CS-305 chainsaw, put it all back together, tested it, and found that the chain brake slipped a lot. Static test (engine off) it seemed to work, but when I started the saw with the chain brake on, the chain ran pretty freely at high idle.
Taking off an inside cover, under the clutch cover, and cleaning off the stuff inside, I found that the top of the brake band was positioned on an eccentric bushing, mounted on a splined shaft. Rotating the bushing slightly adjusted the effective length of the brake band. After reassembly, the chain brake worked as intended on a dynamic test (hitting chain brake handle with chain at full throttle).
Can't say this works for other model saws, but might be something to check.
Philbert
(*The front of the saw is to the left in the attached photo - inside cover removed)