On my '89 LeBaron GT Turbo convertible, the brake light comes on as soon as I start the car and doesn't go off. The pedal basically goes to the floor and unless it's just idling, there's barely enough brake to stop the car. I've checked the fluid level and the reservoir is completely full front and back. According to the Service Manual I have, following the diagnosis flow chart, I could either have contaminated brake fluid (it does look a little dirty, but it has been in there a long time) or the master cylinder is shot. I'm guessing the Master Cylinder. Regardless, if I replace it, the system will get flushed and new fluid.

The next question is how do you determine exact which Master Cylinder I need? The auto part stores (Auto Zone, Advance, Rock Auto, etc.) show about 10 different ones. 4 door/2 door, non-turbo/turbo, etc. I can figure out but they list "Fluid Level Window", which I don't remember any of my cars ever having, 21mm, 24mm, and 7/8", and several that say rear disc brakes 15" or 16" wheels but they even have several of them.

My car's Master Cylinder has " 7/8" " cast into it but it is rear non-vented discs and came originally with 15" wheels (now wearing factory 17" wheels).

Why the hell are there so many applications listed? I know Chrysler and they wouldn't have more than three or so for ALL the FWD cars they made let alone the ten+ that the part stores show.

Anyone have any ideas on confirming and knowing which one I need? (Part number maybe?)