91 integra brakes

alright, i just bought a 91 gs a couple weeks ago. The anti-lock brake light started coming on a couple days ago. it doesnt come on as soon as u turn on the car, so its not just a simple resetting the ECU problem. It comes on a few seconds after i start driving and doesnt go off. I have already changed the pads and it has plenty of fluid, i'm thinking its prolly a sensor somwhere down the line but was wondering who might know a little more about it.

does it make a grinding noise when you start it or b4 the light comes on?

no it doesnt make any grinding noise at all. the light comes on a few seconds after I start moving. The brakes work fine, they just dont seem to be building up a whole lot of pressure to push the pedal back up after i press the brakes. It might be the master cylinder, but i'm really thinking its a sensor somewhere. got any suggestions?