Brake pedal slowly goes all the way to floor

I have a 95 GMC Sonoma 4.3 w code, automatic, 2wd with approx 150,000 miles. My brake pedal was slowly going all the way to the floor. The master cylinder was full of fluid and none of the wheel cylinders were leaking. I replaced the master cylinder with a new one and bleed all four wheel cylinders. The brake pedal still slowly goes all the way to the floor. Please help. Thanks.