1977 240 D.......... Son drove the rear bads smooth.......when replaced it was hard to push in calipher. After some wd-40 it went in. However, now the pedal will go to the floor when depressed. Replaced the caliper and master cylinder. Now pressing on the pedal iis very hard and car does noot want to stop. So I replaced the booster master cylinder from a new model, 83 300 the booster was larger and the master cylinder seemed to fit
problem still exist should I try the vacumm pump

Have you bled all the brake calipers? front and rear?

I had a "brake-experience from hell" weekend myself and know your pain.. turns out my problem > brakes cylinder and calipers would not bleed .. was a defective master cylinder...