My rear passenger side brakes seem to be sticky from a cold start. After a 15 to 20 minute warm up, they unbind and work fine. Could it be a frozen brake line?

 

Could be, when was the fluid last changed?

 

Less than a year ago.

 

Maybe the stiffness is from using dot3 not dot4 fluid...the dot3 stuff is a bit thicker, more common...it is often in shops preassure bleeders...
The fluid in the resiviour (unless it is dyed, like the blue stuff) should be semi-clear...if it is brown-ish, like maple syrup, it is likly pretty old, and moisture ladden.

 

More likly, you pads/rotors are cold....and possibly contaminted with sand or frozen water.....warm them up, clean them up, and they work fine.
What's pads are you using? Different from last winter?

 

Should be EBC green stuff. I'm going to take it in next Monday or Tuesday and have it looked at.

 

Brake fluid absorbs moisture, if it was changed in the humid summer, it can effect the freezing and boiling points (and if you boiled the fluid late in the summer that'll make it worse as well), which would make them more likely to freeze, at lease in some small sections of the line (it's a rubber hose and much of it runs the length of the car with no heat source nearby). If it's the fluid you may also see some clutch feel issues.

With it just being the one side of the rear lines, and also being the passenger side, you may also want to have a friend take a flashlight and see if the parking brake is engaging and dis engaging or if it's got snow/ice getting jammed up in their that needs to be shaken out over a few bumps. The parking brake is a cable that moves inside a sleeve so some of that cable could also be iced over.

 

If we snow on the ground, or had recently washed your car, I might think the E-brake cable was stuck/sticky...so the pads were not releasing...that is why after driving in snow/slush/washing a car, folks with sticks are usually told to park using the clutch in flat area's...not wise on hills, but usually ok in a flat area with a good clutch.
What you may find is that the caliper is partially siezed...and a new/rebuilt one my be a long-term solution. Good-luck!!