Remove the master cylinder lid. Take a convenient finger and rub it around on the bottom of the master cylinder. One chamber at a time.
If you remove your finger and find the tip black with gooey paste, the seals in the master cylinder have decomposed. Wipe your finger and do the other chamber.
No flushing, no farting around, nothing will fix this except for rebuilding or replacing the master cylinder.
Unless you have had a recent brake job, you MUST pop the rubber boots on the rear wheel cylinders and see if any fluid leaks out. If yes, the rear cylinders should be replaced. Check the front calipers or cylinders and look at the brake hoses.
I have EXPERIENCED a master cylinder failure. When the brake pedal hits the floor I'd swear it made the vehicle go faster.
This is no time for amateur hour. I figure when I go to the states, you may be vehicle behind me...