Hello I have a 1996 buick riviera with 130,000 miles and a 3.8 supercharge engine. I bought the car with brake problems and recently I had the car put on the computer to get it diagnosed. Code 15 (not sure)and code 46(Left front ABS pulse-width modulation valve) came up for the abs. The wheels are locked and even though I have new break pads the car won't stop as if I don't have anything. What are these codes and what do I do first?