PURPOSE: To easily and certainly seal the through holes by inserting a seal installation cylinder for the installation of an expandable boot into the through hole on a booster piston, etc. and revolving said seal installation cylinder by a tool and locking the opened edges of the through holes between the locking hooks and the flange on the installation cylinder.

CONSTITUTION: A seal installation cylinder 83 is inserted into the through hole 79 on a booster piston 5 and the through hole 81 on a piston diaphragm 6, allowing the locking hook 84 to conform to the cut part formed into the through hole 79 of the booster piston 5. The short cylinder part 88 of a tool 87 is engaged with the seal installation cylinder 83, and each engaging projection 89 on the outer peripheral surface of the short cylinder part 88 is engaged with each tool engaging concaved part 85 on the seal installation cylinder, 83, and a receiving tool 90 is pressed onto the front surface 5a of the booster piston 5, and the thick part 52 of the piston diaphragm 6 is compressed by pressing the tool 87, and a handle 91 is revolved in the direction of arrow (d). Thus, the opened edges of the both through holes 79 and 81 are locked between the flange 86 of the seal installation cylinder 83 and the locking hooks 84.