1988RX7SE2x2

hello everyone im new and i just bought a 1988 rx7 SE 2x2, its gray and kinda rough, ill just skip my intro and go straight to my problems, and questions, so first when i brake my pedal sinks to the floor but when i pump the pedal it kinda works but sinks again when theres pressure it seems, i imagine its the brake booster right? because theres no leaks on the ground and the pads r new. i just dont want to waste money one something that isn't bad. also i have 119000 miles with just regular maintenance, but i also heard that the rotary needs to be rebuilt every 100000 miles, is that true? i am a mechanic so i know lots too but nothing about rx7s. haha