Loss of normal brakes would be a loss of the right system hydraulics so could you not continue to taxi as the system will default to centre system where the (normally running) ACMP's will supply alternate brakes? Steering as you know comes from the centre so that's not a issue.
If you are down to Reserve brakes there is a loss of both right and centre hydraulics, a loss of centre system (reading 0.0 on EICAS) gives just over a gallon remaining as 0.0 is stack pipe level, and the isolation valves close at about 0.4.
The reserve brakes/steering EICAS message indicates that brakes or steering may not be available.... hence the need for a tow.