Dave the trick with the chamber and liquids is to make sure you remove as much air as possible in the bag before sealing. Sometimes I'll remove a spacer plate inside to raise the sealer above the bag. Lastly, try to place the bag on the sealer so that is can breath. So that when it does boil, the pressure in the bag can escape as opposed to pushing the liquid out.
Also, get the liquid as cold as possible. sometimes I'll stand over the unit and when I see the liquid start to creep toward the opening, I'll press the STOP button and it will seal the bag and release the pressure before leakage occurs.