Having read LOTS of posts on this forum, I've heard it mentioned many times not to lose the o-ring on the master cylinder and/or make sure to not let the o-ring suck into the booster.
I have a 1998 F150 XLT that I bought used some time back. When I changed the master cylinder I never saw an o-ring or any mounting gasket. The new parts did not include an o-ring and/or gasket.
Was this not used on '98s or am I missing something?