[samblaze]

I would check pads all around for wear. The reason I say this is because I had a rear caliper wth an extremely corroded piston that 'froze' intermittently and had caused severe pad wear. Then when it magically decided to unfreeze, the piston had to extend so far out to 'catch' the rotor that my pedal started feeling mushy. Did you do a vacuum test on the booster line? Compression test on the engine? The only other thing I can think of right now is a gently stripped bleeder screw (where it feels tight/closed but actually spits out some fluid under pressure), but then you would be losing fluid which you haven't mentioned so I'm assuming you're not.

 

[samblaze]

This is interesting indeed. I would tend to lean towards corroded front pistons as well at this point. What about ablockage of some sort in the lines (not the short hoses but the long solid lines from the MC/reservoir carrying the fluid to the rubber hoses that inject it into the caliper?

 

[samblaze]

Damn....maybe a machine bleeding would help.