You need to leave the valve open. It will absorb pressure. I use one gas ball lock fitting for two kegs, need to bump up every 12 hours as they carbonate. Make sure you use soapy water on all your gas fittings so you dont lose all your gas if it leaks. Froth up the soap and brush on with a paint brush; examine all connections for growing bubbles. These are leaks, fix them if any . Carbonation: Give it 3 days if you arent impatient. Close gas, relieve keg pressure to about 10 psi. and try some. I use a small picnic tap line inside the kegerator for sampling. If not ready disconnect picnic line and pressure back up to 30 or so for another day. IF ready, leave at 10 -13 psi and connect draft lines. Make sure you dont expose picnic line to more than 13 or so psi. ( or your draft lines once conncected).
Eventually the beer will saturate with co2 and become foamy,after a few weeks. At this point I shut off gas, relieve keg pressure to 0. The beer will pressure the lines enough to dispense, and carbonation level will come down. CHEERS