"I thought you turned the hydraulics on".
The 727 had a famous gotcha where the normal brakes were on the B system which had electric pumps. There was a ground interconnect with the engine pump driven A hydraulic system. If you somehow lost electric power with the ground interconnect closed (several ways for this to happen with gen trips, transition to APU and it quits etc.) you would have to remember that safety wired handle for the air bottle to apply brakes if accumulator braking was not enough. Or, something like this, it's been a while.
Do the late model '73's still have these two legacy hydraulic systems?