You'll need some sort of small diameter punch, scribe, poke etc. Unscrew the brake light switch from the combination valve, then use the aforementioned tool to re-center the shuttle in the valve between the front and rear ports.
Now, bleed your system with the combination valve bleeding tool, like this one.
https://www.amazon.com/Combination-P...s%2C160&sr=8-5
That will keep the shuttle centered while you bleed the system. Once bled, remove the tool and rethread the brake light switch.
You can try the advice you were given, but if the switch already tripped to the rear brakes, it's blocking flow to those calipers/drums and there's not likely enough pressure differential to get the shuttle to move forward.