Tektro disk brake piston

jamie1996

Hi everyone
I was checking the wear on my brake pads today and once i took the brake pads out i squeezed the brake lever not knowing what would happen, the piston popped out along with a load of mineral oil. I now plan to bleed the brakes. I have put the piston back in however i'm not sure if it's in far enough as i can't fit the pads back in. How far are the pistons meant to go in and how can i make it go in further?

The Rookie

Should go back in so the front face is flush with the inside face of the calliper (no point going further or it wouldn't be touching the back of the pad)