Brakes work but pedal is soft

[Patcon]

So it doesn't come up if you pump it??

That was my first thought too

[Cooper260z]

@Patcon nope. If you press the brake pedal when the car isn’t running it feels like a normal pedal, but when it’s running it just goes to the floor and doesn’t change. 

[Patcon]

Does it have stock booster size.

When its running,  if you pump it quickly, it wont catch higher?

[Zed Head]

Put "bad master cylinder" in the search box and interesting threads will come up.  Aftermarket parts...

Most auto parts stores have indefinite returns, and the MC is actually east to swap.  Just a pain to rebleed again.

If you want to get a feel for the mechanicals you could disconnect the return spring on your pedal.  Then you can see/feel where the pedal starts working.  Try pressing the pedal really hard while the engine is off.  The booster kind of takes away the feel.  If the MC is bad you'll feel the pedal slowly sink.  You'll have an image of brake fluid slipping past seals as it sinks to the floor.

[Cooper260z]

When the car is not running, stepping on the pedal provides the normal feeling of stepping on the brake pedal when a car is off, it’s harder to press and stops at some point. The pedal does slowly sink to the floor in this scenario. Though with the car off if I press the pedal as hard as I can, it gets stuck for like half a second before coming back up and a bit of a clunk can be heard. 
 

the booster is stock I believe and pumping the brakes with the car running provides no additional effects. 

Im interested in looking into the piece that Falles off of the push rod since that’s common, I’ll look up how to do that later on when I have time.

Im not ruling out a bad MC, I’m just trying out the things that I can before going that route 

 

 

[Zed Head]

I had the sinking pedal in my car when I first got it.  I remember sitting in the driver's seat pressing the pedal trying to understand where the fluid could be going that the seals were supposed to be pushing in to the lines.  The hydraulic seals and the cylinder were designed to hold, with no leakage.  "How can this be?"  I realized that it couldn't be, the seals must be bad.

https://itstillruns.com/test-brake-master-cylinder-5779577.html

[Patcon]

I agree with Zed, if there's no fluid on the ground it has to be going around the MC seals. The booster just does it faster. You need to see if its going into the booster.

Does it consume fluid?

[cgsheen1]

In your bleeding process, did you bleed the master cylinder itself?

[Cooper260z]

i have bled the master cylinder. 
I’ve put a total of maybe 50 miles on the car but I have not noticed any loss of brake fluid. I’ll have someone step on the pedal while I watch the fluid like that article says 

 

 

[Cooper260z]

so if I take the master cylinder off, this is the view of the booster (first picture.) is there a way to get further into the booster while it’s on the car, to see if that disc has fallen off?

Also, I don’t know if it’s worth noting, but if I pull the rod out I can feel it slide out of whatever is inside there to guide it, and after that it just kinda flops around (second picture) i don’t know if that’s normal 

 

 

[Terrapin Z]

You can just pull that straight out. There is a metal ring with a rubber seal on it that will be loose out of the hole (#1 in the side view pic). The reaction disc (#18) is on the back disc of that rod (#2)

Just judging by the amount of adjustment thread is showing I'd venture to guess the disc has fallen off. I had to remove the whole booster and shake it around until I could fish out the rubber disc. 

I do not think you can open the booster while it is still in the car. Gives you the chance to repaint it too.

Even when adjusting the rod they mention the disc dropping out. I glued mine back on to avoid it falling off again while reassembling. 

I assume you have a FSM?

 

 

Edited December 10, 2020 by Terrapin Z

[Cooper260z]

so if I get the right grip and leverage on the pushrod, I should be able to just pull it out? It hasn’t come out so far from my pulling, and I obviously don’t want to damage the metal ring or rubber seal. 
If that’s the deal though then I’ll do my best to get it out. 
 

I do have an FSM, and it doesn’t mention pulling the pushrod out by this method, that I’ve seen