pressure adjusting piece 82 is pushed upwards to cause the pressure releasing piece 74 the position limiting piece 72 and the supporting member 54 to move downwards thereby bringing about the deformation of the tightening member 48 so as to seal off the air valve 92 in the meantime the first leakproof element 78 is corresponding in location to the outlet of the second channel 20 while the second leakproof element 80 is located between the second channel 20 and the top edge of the receiving space 16 so as to keep the receiving space 16 airtight the two pistons 30 can be caused respectively to compress the first air pressure chambers 32 of the two cylinder chambers 12 and 14 by exerting a pressure on the hand grip 44 in a direction indicated by an arrow as shown in fig2 as a result the high pressure air is caused to flow to the receiving space 16 via the two channels 18 and 20 as the piston 30 is caused to compress the first air pressure chamber 32 the atmospheric air is injected into the second air pressure chamber 34 via the gap located between the piston rod 42 and the cover 15 the high pressure air is then caused to flow from the receiving space 16 to the valve chamber 58 via the second air inlet 64 of the supporting member 54 the high pressure air is subsequently allowed to enter the axial hole 62 of the supporting member 54 via the gap located between the valve piece 68 and the projection 66 it is the axial hole 62 through which.