As far as I know , the safety switch does not shut off any fluid flow.
Getting fluid to the back is sometimes tricky. Assuming the M/C has been bench bled, I would, if possible, crack the line at the safety valve, on the M/C side and bleed until fluid comes out there, and then move to the Y-block at the back and repeat there, and so on.
But you will need to pump the fluid out there with the brake pedal. Sometimes the fluid will move out of the M/C on the downstroke, but for some reason the same fluid will return on the upstroke. This is usually because there is air trapped somewhere in the line to the rear, and it becomes pressurized on the downstroke and then unloads on the upstroke. By cracking the line at the safety switch, this ensures an escape path for the air.
Sometimes a more rapid pedal cycle will get the C-port to supply fluid faster than the fluid can return from the line. Sometimes you have to physically remove the line and use your thumb as a check-valve. Once the fluid gets down to the safety valve, and out the otherside, usually the fluid is well on its way, cuz the fluid is now wanting to siphon.
Once the rear system is bled, the safety switch should self-center.
BTW
the rear brakes are activated one of two ways. When the system is correctly operating, then the rear brakes are operated by hydraulic pressure from the front system. A slug of fluid between the systems does the work. But if the rear system is open, then the piston for the front brakes has a pushrod on it that travels down the bore and pushes directly on the back of the piston that operates the rear system. The point is you will have to stroke the pedal to the floor, or nearly so,to activate the rear piston.Sometimes I have had to open the front system to allow the front-brake piston to travel down the bore and do this job. When I do this I use the bench-bleed hose to recirculate the fluid in the front-brake reservoir. For this reason, on a system where the lines have been emptied, I pump some fluid down the front lines and then close their bleeders. Then I completely bleed the rear system. Then I finish the front.While I am bleeding the rear system, fluid in the front system is shuttling back and forth through the compensating port, forced to do this by the compressed air in the front system. However it is just moving back and forth in the line each time, a few inches. It does produce a geyser each time, so the cover needs to be on and clamped close; else you will have a mess to clean up; another vote for silicon fluid.
