Now I will say this before I reply....I dont have much experiece with the ranger in the braking department......
I have done master cyl and booster swaps on my chevy, my parents escort, my old toyota, and countless different chevy's.......and none of them have a gasket between the cyl and booster. What wears out are the o-rings around the piston, causing a slow leak and a mushy pedal. There may be a rubber spacer thing, but no actual seal. You problem might also be caused by a bad fitting, you aughtta check all those out as well, so you can X those off the list of things to check.
'01 XLT disguised as a '03 Fx4 Level II
Big. Loud. Slow. Bright.