Fluid is the simplest thing to check first. But if that looks good, I expect that car to have "well above average" brakes so it could be the master cylinder or calipers are worn; look for signs of leaks at those components. They might need rebuilding. If you are not sure how to do that work (relatively simple with instructions) get one of those repair manuals for that specific car. For example
Chilton BMW Paper Repair Manual - BMW 635CSI
Fits: 1985 - 1988 BMW 635CSI Repair Manual
Brand: Chilton Paper Repair Manual
Part #: Chilton C1018300
Description: Total Car Care Paper Repair Manual, Model Specific Coverage, Simple Step By Step Procedures For Engine Overhaul, Chassis Electrical, Drive Train, Suspension, Steering And More, Including Trouble Codes And Electronic Engine Controls, GROUND SHIPPING ONLY Price: $27.95
http://www.stanscycle.com/bmw-repair-manuals.html