What is the "problem" which indicates a bad Booster? Either its ABS or Brake Booster.... If it's an ABS fault you may just need to clean or replace a faulty sensor. Could also be a bad ABS unit (not the brake booster). There are a few things it COULD be - but if the light is on in the dash - and cleaning the wheel speed sensors does not fix it - get a Saab shop with a ISAT scanner to pull the codes... (not sure if a TECH 2 w/adapter would talk to a 1990 ABS system)