My ABS lights are on

[OmarA]

I have a 2003 VW Jetta VR6, I bought off a guy on craiglist a couple of months ago and the car was working perfectly. Only thing I had to change was the wheel axel, two tires and the back two breaks. I got these all fixed at firestone. I live in Florida and it's been raining quite hard the past week or two. Recently, like a couple of days ago, I was pressing on my break pedal and it felt like it was like jumping back up at me or like jittering and it scared me so I went to firestone and then they told me my back two breaks were wearing thin and had to be changed and I told them about my ABS light being on but they said they could scan my cars system and see what was wrong for 100 and I said no. So after I got my car back it was fine and the light was off but it still did that jittering thin, but only now and then, it doesn't happen frequently. ABS light came on last night again, but I don't know why the guys at firestone said everything was perfect with the breaks and they did a complete check on my car they said everything was perfect just need an oil change soon