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?