Banging steam heater pipes

[MainMarian]

We have a steam heating system with old radiators. Every time the heat comes on the pipes bang a few times. We have been told this is likely caused by water trapped in the pipes. Draining the system used to fix this, but now the problem persists in spite of draining. We were told that there may be a pipe segment that is not level, causing a pocket of water to remain, or that there may be water in one or more radiators. Each radiator has a bullet-shaped relief valve that usually hisses when the heat shuts off, but no obvious bleeder valve.In addition to draining the system we also tried turning the heat way up for a prolonged time to get all radiators steaming. Neither trick worked. Any suggestions for other things we could do to drain the system completely? Thanks.Paolo and Karen

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