Nitrogen also used due to inert properties in the event of wheel fire & lack of moisture which can cause internal corrosion of wheel hubs. The wheels on a 747 are left out on T/O if a brake has been deactivated. There are two methods of brake deactivation (747 classic) one uses a lock out tool which maintains inflight braking (approx 400ps) so gear can be retracted normally, the other method is for the hydraulics to be toatally blanked of removing inflight braking & the mel gives a figure for gear to remain down to allow the wheel to stop rotating before entering the gear bay (think its about 2 mins). If this method is used climb out is affected so there can be weight penalities incurred. No T/O should be attempted until the brakes are in the correct temperature range.