2001 Toyota Camry Brakes

When I got this car 6 months ago the brakes worked fine. About a month ago, the brake pedal became soft and you had to push farther to get stopped and then you started to need to pump the brakes to stop. I took it to my local mechanic who is a GM Master Mechanic who noticed a small leak in the rear brake lines so replaced them, that worked for about 2 weeks then had the same problem. He replaced the master cylinder with a recycled one to see if that was the problem, but no dice. He says lots of fluid in the master cylinder, fluid going to the rear brakes (bled them 2 twice), and he is stumped as to what the issue is. Wondering if you have any thoughts before I scrap the car.