2000 Chevrolet Cavalier ABS light and engine light

I have a 2000 chevy cavalier, 2.2l I believe. The brake was extremely spongey, I would have to push down almost down to the ground to get it to completely brake. Other than that the car was working great. I took it to a mechanic who bled the line, and they also fixed the front brakes (new brake pads, etc). After it was fixed, the pressure in the brakes built back up and it was working fine. After letting it sit for a night, the pressure in the brakes went out again. I pumped the brake, the ABS light came on and the brake pressure came back. Anytime my brakes have high pressure, the ABS light is on. If I do not hold the brake down when I start the car, the ABS light is not on, and the pressure goes down low until I press down the brake. Now, a check engine light has come on, and the ABS light stays on as well. Why would the ABS light on mean better brake pressure, why does the ABS light and pressure go on and off, and why would a check engine now be on, out of nowhere. A lot of work has been done to this car, it drives smooth, doesn't make any strange noises at all either. Thanks