Brake Warning Light

Looking for some advice. Currently in Little Rock making my way to Houston. Temp here in morning is 16. Prius started OK. Pulled out of parking spot. After first stop, brake warning light comes on and car feels like something restricting motion (like holding car back a little). Brakes felt OK in all other respects (as in they stopped vehicle and then appeared to release). After couple minutes of warm up (slow driving), light out and restriction feeling gone (Tests in hospital today so had to proceed). Light out all day with temps in 30's (drove about 60 miles). Brake fluid OK. Pads/shoes OK. Emer brake was not used. Something is related to extreme cold but cannot id. Car not pulling to one side. Can be trouble in master cyl? Water freezing in system? No other lights/codes. Maybe should take to dealer here but have nothing to show him. Ideas?

 

Updated info. Wife now informs me that several weeks ago (following another cold night), emer brake would not release. Pedal worked but light stayed on and vehicle did not want to move. After warmup, she heard clunk from under vehicle and light extinguishes. Vehicle then fine in all respects. With that, I now believe that problem may be with rear shoes freezing to backing plate overnight. As I understand the manual, if brake system failing, brake light is accompanied by buzzer, which I do not experience (by the by, temps today in 30's, so no brake issues at all). I intend to remove, inspect and clean rear brakes when I return from trip. For Bob: transaxle fluid has 62,000 miles, tires at 42 pounds, alignment I don't know but she drives arrow straight, water in brake fluid I don't know how possible but valid concern, 12v battery about two years old and not tested. Vehicle liberally covered with road salt from 600 mile drive in crappy weather so that could be factor.