initial loss

I keg my cider at 18psi for about two weeks and it's nice and carbed up. My question is, will I always lose the first pint or so to pure foam? Basically, I turn my CO2 down to 5psi for service. My line is 15 feet (3/16") and I serve between 35-40 degrees. And every time, the first glass and a half is straight foam. Then it flows nicely. Any input appreciated.

Your cider is carbed up at 18 psi, which means the head space is also sitting at 18 psi. Turning down the regulator to 5 psi doesn't change the pressure in the head space, so if your beer line doesn't provide enough resistance you can expect gas breakout. You might want to add enough beer line to handle 18 psi...

Cheers!

[edit - corrected the carb pressure] The other possibility is your lines and faucet aren't cold enough...