Had the exact same problem with my 2002 Dodge Grand Caravan. Replaced the master cylinder and bled all 4 wheels on 5 different occasions. Each time the brakes returned to normal but over a period of weeks the pedal gradually became softer with increased travel. Eventually it would go to the floor and have no stopping power. There were several rather mystifying points; first of all very little or no air seemed to be expelled with each bleed. Even when the pedal traveled near or to the floor, there was no red brake warning light and there was no ABS light or fault codes. The culprit was something relatively simple; the lower slide pin in the left front caliper mounting bracket was frozen. It had no in and out movement whatsoever. Normally you would think the end result would be abnormal brake pad wear, but the resulting improper movement of the caliper piston not correctly drawing the outboard brake pad into proper contact with the rotor can and will gradually produce a soft brake pedal. Definitely something worthwhile to check.

 

This link seems to ae about transmission fluid types.