I was checking voltage at the gas valve on a coleman furnace and my reading was 18 volts. What would cause the drop in voltage? I do have 24v into the board.

Check the voltage ACROSS the gas valve relay contacts. It's probably between R and one of the gas valve terminals. In one case you should have 18 V and I'm expecting the 6 to be across the relay contacts indicating that the relay is bad.

Any contact in series with the gas valve is suspect.