In order to correctly replace shock absorbers, you should vent the new dampers before installing them.
Due to the horizontal storage of the shock absorbers during transport, gas and hydraulic fluid are mixed together inside the shock absorber. If you want to replace car shock absorbers without venting the new ones beforehand, the optimum damping effect will only be achieved once the vehicle has sprung up and down several times. This may lead to an increased build-up of noise (hissing).
In order to prevent this unpleasant effect for the driver when replacing the shock absorbers, the piston rod must be manually pushed and pulled several times in the operating direction. This separates the gas and hydraulic fluid from each other again.
The shock absorber has now been vented and will achieve its full shock-absorber effect immediately after it has been installed.