Knock in the brake pedal while braking

Anybody has an idea over last month I noticed a single knock in the brake pedal while braking. The knock comes each time when depressing the pedal a half-way down. The knock is present with the engine off as well.

 

The thread is posted under 348/355 forum. What is it has to do with M3? With the engine off the ABS is also off. The car is not new to me. The issue has started about a month ago. It never occured for years prior to. If you don't know, say that you don't know.

 

'99 355 F1. The issue is present with the engine off also, so don't know if it has to do anything w/ ABS. With the engine off the brake pedal travel is much less, about 1/4 of when the engine is on. And the knock occurs a half of that distance. The source of the knock appearing from somewhere the pedal lever goes into the wall.

When driving if i don't need to brake more and depress the pedal little bit not to a half way, the knock not happening. Appearing it's happening at the same place of the pedal travel to about a half.

I saw many posts in Google for other makes/models. Exactly same symptoms but no clear answer/outcome of the cause and how others had fixed it.

The knock is present every time the pedal is depressed. With the car standing still the pads after the first brake appliance have been pushed to the rotors already so it's not the calipers pistons. A master cylinder or the vacuum booster?

 

The info and experience from Ernest is very useful. I came to a similar observation and opinion appearing something worn off in the master cylinder. Laid down today (more precisely belly up) under the dash (so little room in there) with a following outcome:

- Appearing the banging sound is metallic looks like brake pedal rod is banging at something in the master cylinder.
-The brake pedal rod can be adjusted by extending it to the master cylinder up to 1/2" turning it clockwise. It is secured by a nut which needs to be loosen first.
-I had in mind to extend the brake pedal rod first but then decided to not to change the alignment as I don't have the reference of what the adjustment needs to be so I turned the rod back to the original position marked by a drop of the yellow paint.
-Reading the F355 Workshop Manual Volume 2, section G, I got an understanding that the braking system in 355 is complicated.
-ON p. G19 it says that during normal braking the lever (10) travels a short distance before pushing forward a slide valve (9) to shut off supply from reservoir. I noticed the shaft travel when knocking on something is about 1mm and appearing it's might fit the description. because the car brakes just like usually BEFORE and AFTER the knock taking place. No difference in braking behavior before and after the knock.
-If the braking system is having any issues, the ECU will show the code of the error by flashing the ABS light. The codes are listed on p. G50. Not my case.
-My grommet on the shaft is ripped vertically already but I decided to not to add any grease in there but to address the issue properly. So I decided to drop the car off to a Ferrari shop to fix as they know of what to do. If the master cylinder needs to be fixed if some rubber O-rings need to be replaced there are repair kits on eBay for $86 + shop labor so it shouldn't be too bad.

https://www.ebay.com/itm/Ferrari-Brake-Master-repair-Kits-please-specify-year-make-model-/173823336013?_trksid=p2385738.m4383.l4275.c10

If I attempt to fix it not knowing what I'm doing (more to the point of what the adjustment points are and what are the adjustment values), having a misaligned braking system could cost me:

A) Life
B) Car damage because of an accident of incorrectly tuned braking system.
C) Car totaled
D) A + C

So I decided to go with: E) None of the above


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