2000 durango

Last Thursday I bought the truck. Before purchase the owner was to fix the braking system, which he did. I have the part receipts, and there is evidence of the work. He replaced the master cylinder, brakes, brake lines, and 1 rear wheel cylinder. the car stopped fine for 3 days. Now I have very little brake pedal, and very little stopping power. Any suggestions? I see no evidence of brake fluid leaking anywhere.