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One more thing-- did you try pumping it up with your foot a couple times? I guess you probably did the equivalent of that after so many tries at bleeding it, but after a pad change you usually have to pump the pedal a couple times to seat the new pads, then top off the fluid and it should firm up.
Even though you get no air at the bleeders, that doesn't mean it isn't trapped in the ABS. The scan tool activates the solenoids to drive any air out of them into the brake lines, then you re-bleed to get that air out.
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Almost always, when you have pedal "Fade" (firm stop at first, then a gradual descent to the floor or pedal stop) it is fluid passing a seal. This can occur anywhere in the system, but the only place it can occur without actually being a leak is in the master cylinder. The piston moves (brake pedal descends) and fluid bypasses the seals slowly (unless the seals are truly missing or gone, then more rapidly) allowing further movement without increasing pressure to the system. This all happens within the master cylinder, therefore no leakage to outside of the system.
If you don't have leaks anywhere in the system then the problem resides within the master cylinder, rebuilt or new doesn't matter. During assembly a seal can be clipped, pinched, rolled or cut, all causing fluid to be able to bypass it and stay within the closed system.
If you have air in the system then the pedal will feel spongy, and perhaps with increased pressure on the pedal it can be moved to the floor or pedal stop, but it requires continuously increased pressure to do so. If the pressure is constant and the pedal still moves, then fluid bypass is the only answer.
Check that first, then, if the pedal moves down under constant, not increasing, pressure, and you have no leaks, remove the master cylinder and exchange it at your parts store.
Be sure to "bench bleed" the new one before installation. It is very important that you do this when it is out of the vehicle, as the amount of air you need to move out of the way is huge and very difficult to get to pass all the way to the wheels to bleed out.
I suggest with the engine on. (Only reason is that pushing the pedal becomes tiresome. The power assist makes it easier. It will work either way. Lights are irrelevant.) Slow steady downward stroke on the pedal, ensuring that the bleed valve is closed to start, pressure applied, bleed valve opened, then bleed valve closed, then release the pedal. (If you release the pedal before you close the bleed valve you will likely draw air into the wheel cylinder, which you then have to bleed out again.)(2 people make this possible.) Repeat the above procedure forever! (Well, at least until you're sure that all fluid in the lines has been exchanged.)
You can do it by yourself with a well fitted hose attached to a slightly opened bleed valve. The other end of the hose goes in a jar, ensuring that the end of the hose is submerged in fluid. Then slowly pump the pedal down and up. If the end of the hose is exposed to air, you have the possibility of drawing air back into your system on the upstroke of the pedal.
Also be sure that the reservoir never runs low. If it does, it's time to start over.
But again, if you have pedal "fade", it's not air. It's only air if the pedal is spongy.
It's vacuum assist. If the engine is running, there is vacuum to assist. If it is not running, there is no vacuum. The key position or any electrical connections are irrelevant to bleeding, only the vacuum matters, and that, only in as much as it makes the pedal easier to push.
There is one possible exception to that rule, and that is the ABS unit. The key on or off won't affect it because it doesn't operate that way. The specialty tool Rick99 mentions would be required to activate that system.
I mentioned before that the vacuum assist module should store enough vacuum to operate the system at least one time with the engine shut off. If it doesn't, perhaps the assist is faulty, causing erratic operation. Check that out. It doesn't happen often, but it can. Also, check your under dash linkages just in case.
The answer to that depends on what model and year. Maybe I missed it but I didn't see whether we are talking about a coupe, convertible or sedan. The coupe is a totally different car (built by Mitsubishi), and they moved the unit around in different years.
You can generally find it by tracing a brake line back from a wheel. When you come to a part with a whole bunch of tubes coming out (six) and some electrical wires, that's it. It's usually around the front crossmember somewhere.
As Rick pointed out earlier, you need a DRB III scan tool to bleed the ABS portion of the brakes. Thing is, though, there never should have been air in there from changing the brake pads. Even changing a caliper shouldn't introduce air into the ABS system. Changing the master cylinder would definitely get air into the ABS unit but your posts seem to suggest that the problem arose before the M/C was touched. That doesn't add up right. Also, you state that with the engine off, you have a hard pedal. If there is air in there, the pedal should be spongy with the engine off or on.
With power brakes, it's possible to get the pedal to go to the floor with enough pressure when the engine is on. That's normal.
Have you tried putting the car in gear (with it up on jackstands) and seeing if you've got brakes? If the brakes will hold against the engine, then you may have actually got them working.