Sounds like you have drum rears, which need adjusting; very good chance you'll need to replace the adjusters too. Good news, they're Beetle parts, so shouldn't be so hard to find. Due to the X-split brake plumbing, the rear shoes much engage the drums before you can build up brake pressure. They're also manual adjusting; no automatic take-up, so you will need to learn how to adjust them and keep them adjusted. Or you can convert over to 944 disc brakes.
As for the hard effort to stop, yes, I'd be suspicious of the booster - check first for vacuum plumbing, then see if it's leaking. Another easy test of the booster is to start the engine with your foot on the brake pedal; the pedal should drop a bit when the engine starts. If not, the booster's likely bad.