I have a 91 Cadillac sedan deville and the ABS light is on. My computer module isn’t totally working so not sure if this is an accurate warning or not but the brakes went out and had to use the e brake to help me park. How do I know if this is the abs system or master cylinder that is bad. The brake fluid is full.

 

I'm sure you've figured this out by now.

If the brakes "went out", you've got a leak...somewhere. This can be an external leak, where brake fluid escapes the plumbing and becomes visible if you know where to look. Rusted brake plumbing, leaky wheel cylinders/calipers, popped rubber brake hoses, etc. The fluid level in the reservoir will drop.

The other kind of leak is internal. There's no visible leakage. This is generally due to failed seals inside the master cylinder. The fluid level doesn't go down, it just doesn't get pressurized by brake pedal movement like it should. The fluid level doesn't drop except slowly, to compensate for disc brake pad wear.

Wild Guess: You have at least two separate problems--the fault(s) causing the ABS light to come on, and the fault(s) causing the loss of brake effectiveness. It's possible--but not at all likely--that the ABS is causing the brakes to not apply.

I expect you'll need professional diagnosis. And it's not likely to be inexpensive.