Mitsubishi Repair Problems? Ask a Mechanic for Answers ASAP
Unfortunately when this happens you are nearly always going to be ending up replacing the HBB (brake booster assembly). This is a common weak point on this model.
What I would want to do first is check all of the fuses both in dash and under hood, and including the larger fusible links. Make sure none are blown, even if they aren't marked as a related circuit.
If the fuses/links are all OK, identify the two HBB relays under hood (identified by an exclamation point on the relay covers, and are right next to each other) and have them tested.
If the fuses/links and relays are all OK, then you are nearly certain to be at an HBB issue. At that point you would want to get it to a shop with an HBB compatible scan tool that can read what codes are present in the ABS system (different from an OBD scan) and potentially condemn the system. Normal faults that will result in an HBB replacement are any pump related faults, ECU faults, or pressure sensor faults. Unfortunately those are the ones that are seen most often too. Most other faults that you get will not cause you to lose braking power (pedal stuck up)... that is pretty much always going to be an HBB issue be it pump or ecu failure; but you must verify the fuses/links/relays first just in case.