The brake failure light is run by a switch that detects a difference in fluid pressure between the two brake fluid "circuits". So if one springs a leak the sensor will trip. Also if one has a rubber hose go soft which allows pressure to sag on that side.

One circuit runs the front upper pistons plus left rear pistons. The other circuit runs front lower pistons plus right rear pistons. Front calipers have four pistons each, rear ones have two each.

Could be a major air bubble and you need to bleed the system.

Could be the switch has failed. Could be the wire to the sensor is grounding on some convenient metal - go take a look. The sensor is on the brake distribution block (aka "octopus"), located on left front inner fender wall, pretty much directly below the master cylinder. Has 8 brake lines: 2 coming in from master cyl and six going out to the calipers. Octopus also has the wire lead for brake failure sensor.

If the switch has failed you can replace it. See fcpgroton.com. The item might be in "brakes" section or in "electrical sensors". However there's a sliding piston thing inside the brake distr. block that sometimes fails and so replacing the switch won't fix it if the problem is in the piston.
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Sven: '89 245 NA, 951 ECU, open-front airbox, E-fan, 205/65-15's, IPD sways, E-Codes, amber front corner reflectors, quad horns. Wifemobile '89 245 NA stock. 90 244 NA spare, runs.