That's what I was going to say. If you have the system apart you will need to bleed all 4 wheels anyway. Might as well replace that rear flex hose at the same time and remove a potential weak spot.

Single circuit brakes have no room for error.

If the car has sat for a long time I'd suggest spraying some penetrating oil (PBlaster works quite well) on the bleed valves every day for about a week prior to trying to open them. Crusty old bleed valves are cursedly easy to break off when attempting to open them for the first time in a decade.