question for you guys. are you letting the pads wear down almost to metal before replacing them? this could be the source of your issue if all you have done is change pads/rotors there should be no air in the system and no need to bleed, however if you let pads wear almost all the way to the metal backing or all the way to it before servicing this could be your issue, as letting them wear too low for extended periods over extends the caliper piston to apply brake force when needed, then when it goes back in there is not enough fluid in the system to keep the air in the reservoir out (yes there is always an air pocket in your reservoir) this balances the pressure, and that's why there is a max line lower then the top, so you have this air pocket. well when you over extend that caliper piston it has to come back to somewhere and it will typically come back all the way in (unless this is done too much then the caliper breaks but that's another story) and since the system is closed it fills the gap with something and this where you get the air sucked in as it sucks down more and more fluid to the bottoms of the system during use (calipers) it has less and less to push back as your pads wear when the pistons retract as the fluid gets below the min line you then get little bits of air pulled in with the fluid as it has to fill the gap and air is right there so it sucks air in as it does not have enough fluid to fill the void, then you get the spongy, soft pedal and lack of braking power.
I had this issue on my '11 GT, when I bought it I knew it needed pads (61k miles, factory pads where shot, and they where factory, I know I changed them myself)well 61k is a loooooooong time on one set of brake pads 30-40k is typical wear life of a good pad (replacement before it wears to metal backing or close to) well when I test drove it all was good. Changed the pads and rotors 3 days later at all 4 wheels and pedal goes soft, so I check my work, everything is in right and tight cant be that. So I decided to check the fluid levels again, well once you get to minimum level on the fluid(with no leaks) its time to change pads just as when you put new ones on it should come back to max. So I check my fluid level only a hair above where it was when I started (right above min) so I knew right then bleeding would be needed tried the traditional front to back still had a soft pedal then I remembered my 08 needed to be bled cross wheel as the system is tied opposite front to opposite back wheel so I tried that and pedal got better, did it again, still softer then I would like but getting there, again and after the 3rd time the pedal got firm before I even added fluid (while bleeding) I knew all the air was gone, closed er up, topped off the fluid 1 more time and went for a ride, pedal is nice and firm car stops on a half dime now.
so just a word of warning keep an eye on your pads, the absolute easiest way to do this is to make sure your fluid is at max when you change them and you have no leaks, then keep an eye on your brake fluid level when your checking other fluids if its getting to min you need pads and pads and rotors are cheap insurance against brake failure, so I recommend checking all 4 wheels when your doing any brake work im so anal I even change both sets of pads and rotors(rotors are so cheap you might as well replace instead of turning and/or reusing) each time I do brakes just because I cannot stand having to do brakes once then 2 or so months later have to do it again for the other set, might as well get it all done at once. plus im anal and ocd about car upkeep so having all that done eases my mind for a little bit
another tid bit, DOT 3 and DOT 4 fluids are compatible with each other and can be mixed, however if your car calls for DOT 4 don't use DOT 3 as it has lower temp handling, but our cars call for DOT 3 so subbing in DOT 4 will get you a higher temp handling fluid that should last a little bit longer and DOT 4 fluid does not typically cost more then DOT 3 However do not use DOT 5 as DOT 5 is a silicone based fluid and will gel up in our system if mixed (not good, and will not mix with older DOT 3or DOT 4) if you want to even try to use it (way not needed in these cars) you will need to flush all the old brake fluids out of everything and I mean everything and they must be spotless so not worth the cost and labor of doing it, however if you can find DOT 5.1 (very hard to find in stores anywhere, even online its hard to get) it is compatible with DOT3 and DOT 4 but for the added cost and effort to find it DOT 4 will serve you fine, we are not running 300 laps at Watkins glen in 700hp cars just driving a 170ish hp 4 cylinder mazda around.