83 300D W123.
The brake pedal has been going way too close to the floor for comfort. The pads in front looked bad so I replaced the calipers with a loaded set of rebuilds and pressure bled the system. No improvement whatsoever. Rears. Not looking great but had a fair amount of pad left. If these calipers were not doing their job, could this lead to a low pedal? I am suspecting the calipers are frozen. Why?, because the rotors, although not very rusty looking, don't appear to be wearing evenly. If those that know better concur, I'll just replace them too. Is ther any reason to suspect my master cylinder? Any way to test this thing on the car? If I have to also replace the MC, should it be bench bled first? All four rotors are in at best, fair condition. If I do the rear calipers, I'm going to replace the rotors since I have a new pair laying around. The brake system has been regularly flushed, about every two years.
Advice sought and greatly appreciated.

Replace the master cylinder and you will be fine. Try to keep to a minimum how much brake fluid you spill on the surface underneath the booster because it will take the paint off. Make sure you bench bleed the m/c prior to installation.

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Jim