try just taking the cap off,, make a rubber seal from an old inner tube or something the exact diameter of the hole then sanwich it in place with the cap,, this should seal it tighter if it is the cap but only run it until the fan comes on then shut off and let it cool. if it stops the coolant loss its the cap thats faulty 
incidentally if it was the water pump seal you would lose coolant through the weep hole underneath onto the top of the exup valve and you would see steam and smell it
incidentally if it was the water pump seal you would lose coolant through the weep hole underneath onto the top of the exup valve and you would see steam and smell it