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I'm trying to get my 3rd valve slide on my Meinl Weston 18 to be able to move very easily while playing. I've tried again and again putting slide grease on it, yet I can't seem to get it to the point where I can move with real ease. I struggle to pull it up at first then it moves easily.

Could be bad slide alignment that needs the attention of a brass repairman.

OR

Sometimes slides stay extended a bit for long periods of time and crud builds up in the exposed lower portion of the outer tubes. Try cleaning the inside slide tubes with a bottle brush and some dishwashing soap/water. If the problems persists, you might try swabbing the outer slide tubes with a cleaning rod with a rag soaked in white vinegar. If that fixes the problem.... the rest of your horn most likely needs to be chemically cleaned, too.