2007 Honda CRV 195,000 miles AWD. I am having an issue with a low brake at low RPM's. I brought it into a local mechanic and they bled the brakes and said it was fine. After a week the problem came back so I changed the Brake Master Cylinder. Now i have a great brake pedal when I am driving but after I coast it goes almost to the floor. I bled the brakes using the buddy method until I was blue in the face thinking there was air in the system. Could this be a Brake Booster issue?

Also, if I get on the brakes hard they react well but if I push them down slow at low RPM's they catch low. Please Help!