^Yes, unfortunately it sounds like the ABS module is messed up internally. This is exaclty what happened to my dads STS (95) a few years ago. Fluid was staying full, brakes spongy, etc....except he had no warning lights. If I recall correctly the mechanic ended up renting some special diagnostic machine before they could identify the problem.
Then he had trouble sourcing a new module...in his case new was very expensive (like 2k expensive), and we bought and checked 2 or 3 used ones which also turned out to be junk. Eventually got lucky and someone wrote off a low mileage car and we got the module from the yard right away. All that said, I don't think you'd have as much trouble finding a replacement....Cadillac's tend to make everything 10x more difficult.

See I told you to delete it LOL

I have a 2000 Cadillac with the same bad module and the brakes work perfect except the ABS/traction control doesnt work. I dont think being "spongy" and no "brake assist" has anything to do with it

I think you may have a small leak in one of your lines,that fluid had to go somewhere.

There isnt any leak lines it hasnt lost any fluid since i filled it back up. Im positive its in the booster as there is no where else it can go. I just havent had time to look more into it, been very busy with school.

IF the ABS module is bad, it is usually because it has somehow become "flooded" with fluid. I forget the details, but there is a section of it that isn't suppose to have fluid in it....Once fluid gets in there it is basically junk. We ended up driving to yard, pulling these modules and checking to see if they were "flooded". So I guess it's possible that the missing fluid is inside this module.

Doesn't take much to check out the booster though, so I would start there. If not, start reading about the ABS modules.