Okay, so since i've had my truck, my ABS and brake light came on. I was told there was a leak on the rear drivers side wheel, but i don't see it and theres nothing on the ground. The brake pedal is very spongy, and when i apply it, it skids other rear, and if i push harder the two fronts. I can't seem to locate the leak, and my fluid is low. I haven't added more yet because I'd like to see if theres a way to locate the leak first, or some time of additive i can add to see. Thanks.