I have a Cad. Fleetwood, 8 Cylinder, 4-2 lit. has soft (swishhy) Brake pedal and both the brake and the antilock brake light comes one only when the brakes are applied–and does not stay on.
The brake fluid was low but the problem persists even after the fluid was full. There is no visible leakage anywhere that I can see..
You could have a bad brake master cylinder, but you better get a mechanic to check it out first before you just replace it as a guess….its kinda a pricey guess. I would also pull the wheels off and check all the brake parts for brake fluid leaks or wear.