I'm getting a bit frustrated too. After the second replacement of the water pump of my 1299S last autum (including filler cap of radiator) and about 300 km riding directly afterwords with no more coolant coming out of the weep hole, unfortonately the problem came back after the first ride this year.
After a ride of 135 km, back home and bike on the side stand, coolant leaked out again, not only one small drop, but as much that it went all the way down to the bottom end of the alternator cover and stayed there as a big drop.
During the second ride, a really big drop came out the weep hole during a brake after 120 km, and again after 40 more km back home. I could watch it myself with a flashlight coming out of the weep hole after a minute of engine off.
I have no idea why the problem is back again. I can't belive that all three water pumps do leak such an amount of coolant to be accepted as normal after only 200-300 km.
I got back with the problem to my dealer waiting for an info what to do next. Changing the water pump again is not promising a final solution I guess.