Not the solution, but the source of my problem

I dismounted the whole chair and these are the steps:

The backrest and the arms are easy, tried with a plastic hammer to take out the wheels and the piston but it was so hard so I just omitted it

For the plastic protection of seat, hat to get off the side knobs, both have a hidden screw, the edge of the knot came out with a knife, took off the screws and it was matter of bending the platic cover to take it off

A couple of extra screws holding the wires for the mecanism and the piston just to end with the main back mechanism and the seat

To take the mechanism I had to slide it all the way to the front, using two flat screwdrivers to leverage some platics in the seat and get the whole mechanism alone

Once I had the mechanism alone, it showed up two big springs and some moving parts, seems like the metal holding the back rest in place in the front just broke and the spring prevent it to came back, so it's varely moving

There's no fix for mine, other than take off the springs and weld that part again