I trained a horse a while back that came to me with a capped hock from kicking walls. He never showed any sign of pain or lameness, no matter what I did with him; loping circles, reining type spins, hard stops, etc.
Like Natisha said, a capped hock that is not fresh is just a blemish and cannot be "drained". If they just injured it recently, there may be some edema to drain but often, the swelling is permanent.
Consult with your vet and have them do a check. That will be your best bet for definitive answers.