I already do a search and look that the common issue on this problem is
if you have unequal pressure in the front and rear braking systems. Could mean you have a leak in either the front or rear, air in the system, etc. The light is activated by the pressure switch in the block attached to the right side of the ABS motor pack.

So, I think that maybe I had air in the lines. Do it's necessary to bled the front system and then rear ? Or bleeding only one is ok ???

 

You need to start by bleeding the ABS unit. There are two bleeders. Then work from the point furthest away from the master cylinder.

 

i've had promblems with my firebird, i needed to replace the wheel hubs. if bleed the brakes doesn't work, and you get a a sound that'll increase with speed coming from your wheels, it's probably the hubs. i just had to do both of mine, have a good way to get them off if the hubs rust against the street nuckle