1990 Fleetwood Brake Problem

I have a 1990 Cad. Fleetwood (4.5 engine), 150,000 miles (with approx. 70,000 miles on motor). When I attempt to make a very fast stop, it feels like the brake pedal pushes back at me and I have very little actual braking power. The red brake light comes on, as well as the yellow ABS light, then both go off after 2-3 seconds. If I lightly (gradually) step on the brake, it seems to work just fine and the brakes have full power. Is this possibly an ABS problem? Can I bypass the ABS completely? My mechanic couldn't find the problem and told me to go the the Cadillac dealer. This situation could actually be dangerous, as when you hit the brakes hard, they just ain't there.