Window mill hydraulic line connection

A piston has a hydraulic line going through it and is held in position with one or more shear pins or similar breakable retainers. The piston blocks the flow to the window mill circulation ports until a pressure level is reached that breaks the breakable members to allow the piston to move. The piston moves into a larger bore to allow bypass flow through lateral openings. The piston is captured in the larger diameter chamber after passing the lateral openings. Piston movement pushes out some of the high pressure tubing through the window mill where initial rotation of the window mill along the whipstock face causes the extending tubing to be ground off. A rupture disc and high pressure hoses are no longer needed for a significant cost savings from prior designs.