Try opening a faucet before turning the pump on. Check your pump fittings and see if they're loose. Loosen fitting on output side of pump, have helper turn pump on and off, you might get sprayed a little so have towel handy. Failing those suggestions you may need to buy a new pump or rebuild yours. You may have a valve on input side to draw antifreeze, check it too. Hope this helps/fixes your problem!
Dave & Myra
One day your life will flash before your eyes,
make sure it's worth watching!