I have left knee patellar tendonitis and a baker's cyst. It was controlled with os ban and activity mod, but now nothing helps. Time to drain/treat?

Maybe. You should have a follow up complete workup, with MRI included, to make sure there are no other factors contributing to your knee pain. If there are no other complications, then drainage of knee and steroid injection may be helpful.
Baker cyst a symptom. I seldom drain baker's cyst, and i'm skeptical that this helps. The cyst is a symptom of what is going on inside the knee. If conservative treatment isn't helping (you didn't mention nsaids or other medications), perhaps it is time for an MRI for further evaluation.

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