Hi, I have been helping my friend bleed the brakes on his 2007 Volkswagen Jetta. We bled the brakes and the pedal firmed up very nicely but in about a week and a half the pedal has gone back to being as squishy as before we bled the brakes. Calipers are all only a few months old if that helps. Thank you!
The master cylinder may be leaking internally and/or there is air in it.
Test: pump the brakes and hold with pressure on the brake pedal. What does the pedal do?
If you have ABS brakes, then there could be ABS problems.
Best to take it to a professional brake shop and get it diagnosed.