Brake Light

A couple months ago I listed a thread because my brake light came on. I made sure the emerg. brake was off and all that biz. I got a few replies back saying don't drive until I get it looked at etc. Anyway, the long and short of it is that I brought it to the shop and they assessed that I needed a new master cylinder and so I had it done. The brake light went off and everything was cool. Recently the light came back on...I brought it back to the shop...they said it was something called a proportioning valve switch or something which tells the car when the breaks need service. Does this sound right? If so where can I get the part for not too much money? Any suggestions on if it is something else would help as well. They did look at them and said they were safe and working properly.

Thanks everyone