On all three versions of the 1290-QUAt pump head the primary head the outlet valve has a very tiny and delicate gold seal inside (Seal P/N G4220-20020). The gold seal goes on top of the thin SS tubing (heat exchanger line) sandwiched inside and held by either a Torx 8 or slotted bit on top of the outlet valve (under the cap). There is no reason to fiddle with this connection, but many users do, and damage it in the process. When damaged, it will leak. So I recommend you first check that this valve has been correctly assembled, with the gold seal in the correct orientation, fitted and torqued correctly. Also the valve nut must also be correctly torqued to the primary head.
These types of services were never meant for customers to perform. Agilent wants all of them to be done by their service dept. That is why their "solution" is to just purchase a whole new pump head assembly when one wears out or is damaged by the end-user.