piston rod 13 the upper surface 24 of the seal member 15 engages with the inner surface of the cap member 16 thereby maintaining the fluid tightness of the chamber 18 thus the seal member 15 surrounds the piston rod 13 and is urged to abut with the inner surface of the cap member 16 according to the invention an annular cavity 25 is formed between the cap member 16 and the seal member 15 the cavity 25 is defined by an annular recess 26 formed in the upper surface 24 in the embodiment of fig2 the bottom surface 27 of the recess 26 is generally parallel with the inner surface of the cap member 16 the cavity 25 is connected permanently with the outside of the damper through an opening 28 formed between the central bore of the cap member 16 and the connecting rod 13 passing through the bore gas under high pressure is enclosed in the upper portion of the annular chamber 3 a mounting ring 30 is secured to the cap 5 the hydraulic damper having the construction described as above is mounted between e g the sprung mass and unsprung mass of a vehicle by means of mounting rings 17 and 30 fig3 and 4 illustrate the process for filling high pressure gas into the hydraulic damper of fig1 firstly the upper end portion of the damper is inserted into a filling container 35 of a gas filling apparatus with the mounting ring 17 not being mounted on the piston rod 13 a chamber 36 defined in the container 35 is sealed by seals 37 and 38.