My 85 300TD has been showing a lack of response to the brake pedal until about the 3/4 way point. It just goes 'pffff' when the booster should kick in. I have to pump the brakes once or twice for it to respond normally, or push the pedal all the way to the bottom.

When I first got the car, the brakes would cut out randomly, and after a month it magically went away, with no effort on my part (other than filling brake fluid to the MAX, but it wasn't horribly low either). It hasn't cut out once since. The brake system was probably neglected for a long time.

I recently changed the rotors and pads, and now noticed the rear calipers were stuck. I resealed the pistons and now (after 3 full brake bleeds, using my Motive bleeder, and no visible air bubbles, the brakes sink to the floor before responding. I don't think the brakes ever worked normally, so I'm thinking there are bigger problems.

There are no vacuum leaks I can tell of recently. There used to be but I fixed them and now the ignition cuts off immediately (unlike before).

I wanted to see if there's any way something is wrong with the master cylinder or the brake booster, as the pedal sinks to the floor (slowly) unlike my other Benz.

What are the signs of a brake booster that simply is not working at all?

I've read that there could be a bad O-ring on the master cylinder. With the power bleeder, I'd imagine a failing O-ring might give out completely under 20psi. Do I have to drain the master cylinder/reservoir to properly check the O-ring?

How would I be able to see if fluid is in the booster, and/or see if the booster is faulty? Do I have to pull the booster out entirely, or can I just see with the master cylinder out of the way? I've also read that there's a point where total booster replacement may be in order, and I'm curious how I could tell. The FSM recommends if there is more than 100cc of fluid in the reservoir.

Is there a point where the master cylinder can fail?

i think you take a piece of string and drop it down into the booster after removing the MC to see if any brake fluid is in it. if the booster is not working the brake pedal would be very hard to press. i would suspect it's probably a bad MC

Observations:

The vacuum peaks out at 20 Hg, and pressing on the brake pedal drops 5 Hg, then releasing brake pedal drops another 5 Hg. After the pedal is released, the vacuum builds back up (even with foot off accelerator). However, comparing my 300TD to a 300SD that has great brakes, the difference is that the problematic car won't build vacuum (though it stays steady) until I let go of the brakes. On the better car, the vacuum builds even while the brake pedal is being held down.

When I have the car off, the pedal goes about half way before it sticks, and when I turn the car on, the vacuum drops 5 Hg (as normal with brakes being held) and then the pedal slowly moves closer to the floor. The vacuum doesn't drop any more. I'm not sure if the pedal not being able to hold resistance means an issue with the booster or the master cylinder.

On my other (better) car, the brakes stay firm even as I hold the pedal down.

Update:

I removed the master cylinder and observed the O-ring was worn/expanded. I took a working master cylinder from a parts car and swapped them, and now the brakes seem to be a lot more responsive.

It still requires pushing down 1/4 of the way before brakes engage, but it responds quite rapidly once it starts resisting. The pedal no longer sinks as I hold it down. I could be wrong, but the O-ring would explain the pedal sinking (ie leaking air), as it's supposed to seal the air in the booster. I probably just could have replaced the O-ring, but I didn't have a spare one on hand and the spare master cylinder was already around.