From the description it appears you've lost brake booster assistance - what was the reason you replaced the rear brake discs & pads? was it wear or an issue with the brakes?
If you were able to bleed them (not usually needed if you've just replaced the brake discs & pads) then the fluid circuits are probably OK. Can you take a photo of the vacuum pipe from the brake booster to the vacuum pump, specifically the part next to the engine cover - it should tee into two pipes that go to the vacuum pump.