Hi, exactly what is the brake warning ? I'd bet its brake light failure, yellow triangle

and message check brake lights?
If it is this and its intermittent and the bulbs are known to be ok the check the well known rear hatch wiring issue at the flexible going to the boot lid?
Why do you suspect the slave cylinder poor gear selection etc or can't select at all? Has the level in the clutch fluid fallen?
Can you select the gears no problem engine off?