Take a cheap straight blade screwdriver and, using a torch, or even a gas burner flame, heat the tip up until it's bring red. Stick it in the slot, gently push the tip into the plastic until it's 2-3 MM deep. Now take a cold wet rag, and cool the end of the screwdriver, and the valve. Pull the screwdriver out, admire your work, and be amazed at the new screwdriver slot you made.