The bolt is not going to get better by itself, The sooner you replace the better, What happens next time ? What else does it effect ? Change it out !!!
Make sure the hex wrench fits the bolts tightly, Insert the hex wrench and tap a few times with a light hammer, Then turn them. They will come out, You take the chance of stripping the hex more that breaking them off.
I had a 98 GS300 FOR 225000 Miles, I went through 3 Master/ABS cylinders, I think there is a subject on using a brake booster from a SC430 Check into that.