I’ve owned my 1986 740 wagon for around 4 months and the brakes have never felt as responsive as they should be. I took the car to the shop to get it inspected for a road trip to college and the technician said my left rear caliper has been closed around the rotor and the pads have 5% life left in them. The shop replaced the caliper and now the brake pedal isn’t spongy. Around 2 weeks later I completed my road trip from Seattle to Phoenix and the brake pedal is the spongiest it has ever been and a few times in bumper to bumper traffic the pedal has gone to the floor and the car will start rolling. Sometimes if the brake pedal is pumped the feedback is better. I tried to take the cap off the brake fluid reservoir but the cap won’t budge. I shook the reservoir and there is fluid in the container.

 

Air in the system , get it re-bled.

 

as noted rebleed then look around for leaks (hoses/lines, calipers). If no leaks are found and the sponginess returns, I'd suspect a leak in the master.

 

25-30 year old master cylinder piston seals do tend to wear out ....