Updated: May 20, 2021
If you need to work on your chair and there's any problem with a motor or the hydraulics, you want to be sure to brace the chair.
A simple frame built of 2 x 4's is an effective chair brace and will support the chair to prevent collapse which could lead to injury. The wooden supports should be placed directly below the chair arm supports or a similar solid portion of the chair frame. With a cantilever-style chair you should be able to fit a simple "U"-shaped brace under the frame at the seat of the chair.
A 2x4 frame as illustrated should prevent the chair from descending so you can safely work on it.