IC,
If you can make the engine run different when you seal off the breather inlet tube, then your Hydrovac has a leaky piston seal. The breather is on the fresh air side of the piston. When activated the engine vacuum sucks the air out on the engine side of the piston and air rushes in from the fresh air side. The two should never meet, the piston and it's seal keeps them separated. Sounds like a rebuild is in order for that as well.
Mike B
