Cheap-Vacuum pump problem

Hello ,
I bought a year ago a basic 100 euro model vacuum pump and i have 2 problems with that.

  1. when it works i see oil in the fitting of the sanction . Why is this happening ?

  2. I have put between the pump ant the switch a piston ( 2 litre ) to act as a buffer and although the pump by itself could give a -1 bar reading now with the piston connected can go until -0.9 bar reading. is this normal?

  3. When connected on the line with the vacuum switch and the pump reaches the desired vacuum stops to work. Then … sometimes - not always - losses the vacuum very quickly .And it works again of course… It wouldnt do that before…