Worn front brake pads=high pedal effort
 

I'd been trying to figure out why the brake pedal effort on my 77 300d was so much higher than the pedal effort on my 85. Turns out it was worn front brake pads. Brake wear indicator had only flickered very recently but had noticed the high effort back in January when I drove the 77 after driving the 85 for a long time. Immediately noticed it took twice or three times the effort on the brake pedal. Initially thought it was bad booster or vacuum pump. Rebuilt the vacuum pump. No difference. The brake light wasn't coming on and I had about 20k on the front pads, so I didn't think it could be a pad problem. In addition, I still can't figure out why worn pads increases the pedal effort.
But they must. I replaced the pads this AM and pedal effort returned to the same level as the 85. Pads were worn down to the indicator drill hole with about 1/8" left on the thinner outside pad.

I think the pistons in your calipers may be binding a bit. The front pads on my SDL have almost 30k miles on them, and are near dead, I'm waiting for the light. But the braking has not changed, I was doing some heavy braking today and the pedal effort the same even when the brakes were real hot.

Perfromance seems to have not changed either.

High effort
 

Some possibilities come to mind:

1. A caliper is stuck or sticky, increasing temperatures on that pad & causing it to glaze over (hard surface), therefore gaining a slicker surface, therefore needing more pressure to stop. I have also done this without a sticking caliper in my old motorhome by using too much braking in the mountains & frying the pads (& the calipers, boiling the fluid, etc). I have pulled pads that are so glazed & hard they ring like bells if you tap the pad surface with a screwdriver.

2. Some pads (I seem to dimly remember) have 2 compounds on the friction material: mostly softer for good braking, and a thin layer of harder stuff for bad braking to indicate a pad change is called for -- before running out of pad & scoring the rotor.

I've never owned a car before that got high pedal effort when the front pads were worn. I don't really understand what is going on. Why would the pistons be binding. The pads weren't glazed.

As the brake calipers age the pistons get pits in them. When the pads are worn the pistons are really extended and they can bind.

When I jacked the front wheels up, spun them, and stood on the brakes, they stopped smartly without a huge amount of pedal effort.

Yeah but that wont tell you anything. With the wheels just spinning like that no force at all is needed to stop them. You can stop them with one hand.

I have had the same thing happen to me on my 77 300d, 81 300td, and 83 300sd. It appears that when the pads wear down sufficiently, the pad's metal backing plates can come in contact with the leaf spring that pushes the pads in radially and you are trying to compress the leaf width-wise. This results in the load being transfered from one pad through the spring, and into the other pad instead of all of the load going through the disc. I hope I am saying this clearly :). As stated before, I have had this happen numerous times and each time I waited until the pedal effort increased, the pads were in contact with the spring.

That makes sense. I didn't look closely enough upon disassembly but it seems like the best hypothesis yet. It took some prying with a screwdriver to get that spring out so it was probably right against the backing plates.
It seems like a design flaw.
It would also explain why I thought for a while that perhaps the rear brakes were the only ones working. I had a sense that the car was not 'diving' enough during hard braking, making me suspect the front brakes were the ones not doing their job. The spring limiting the amount that the pads could squeeze would account for that sensation.

Though I had heard of poor front brakes with low pads I could think of nothing that would limit the piston travel. I really like that spring ideal as it makes common sense.

Another observation of mine is that this situation gets worse as the rotors get thinner. This will happen with more pad still left on the backing plates.

Good point.