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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Update:

I had bled all 4 calipers - knocking was still exactly the same. Then I had found a generic procedure couple weeks ago for bleeding the brake master cylinders. I had replaced the master cylinder (the plus is the 355/360 master cylinder has both bleeding valves on the top of the cylinder and the expansion tank not on the top of the cylinder but aside connected by 2 rubber hoses so no bench bleeding was needed. Before removing the original master cylinder start the engine and press on the brakes make the pads to secure the rotors so when bleeding the master cylinder it will be no need fluid to go in the copper pipes from the master cylinder to the brake lines dragging bubbles. When the new cylinder is in place I needed to connect clear vinyl hoses to the bleeding valves and then opened both before pouring the brake fluid to the reservoir. Then poured the fluid in and watched the bubbles going in the bleeding hoses as the air was coming out from the master cylinder while the fluid was getting in. Then waited 15 mins letting the bubbles to settle. Then closed one bleeding valve. Turned engine on and pumped the fluid having the hose the other end coming back to the expansion tank. But the end of the hose you need to cut so it will not dive to the fluid otherwise you will be bringing bubbles back to the fluid. I've read on one thread someone recommended to perform the operation with the engine off but if you have the engine off it will be hard to manage the pressure on the brake pedal when pumping as of when it's enough so you can rip off/damage the rubber sleeves on the master cylinder if you press on the pedal too much. On the working engine the car stands still so ABS is not bypassing the fluid as the wheels don't move (and lock for the ABS to start bypassing fluid), and the vacuum booster working gives better feeling when to stop pumping. So, then turned the engine off waited another 15 mins. Then repeated the procedure with the same valve 3 times. Then did the same with the 2nd bleeding valve. You will not be getting all of the micro-bubbles out. After finishing wait a day or two then put a clear hose to one then another bleeding valve let the rest of the bubbles out - they will be invisible as over the day or two the air will accumulate by the inner top-side of the bleeding valves.

So I replaced the brake master cylinder and the knocking in the brake pedal was exactly the same. No any difference. Then next day I had put the original master cylinder back in.

So then I brought the car to a Ferrari shop and they spent a great amount of time trying to find of what causing the knocking and finally found in my case ('99 F1 M5.2 Bosch ABS) the cause of the knocking was the brake line pressure switch, p/n: 154309. They said they never have seen before this switch to fail. They replaced it and the knocking went away.
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It is leaking but very little. There were no braking fluid anywhere in the front trunk compartment around the plug. If you see from the photos, inside the connector there is a rounded hole the braking fluid was coming from.

I passed of what you said to the guy who fixed it. He said he doesn't know of what the switch is doing but both of us believe in what you said. I also noticed my 355 never held the nose on braking always nose-diving. But, I never had another 355 to compare to. So maybe the dead switch was indeed not sending the signal to Bilstein ECU to harden the front shocks on hard braking.

The guy said as the brake pedal travels down and the pressure in the braking system building up to a certain point, something was clicking in the switch. Although when I was hand-pumping the brake pedal by hand I had a sense something was clicking in the master cylinder/booster. But I do know, the liquid passes sounds very well so maybe the brake lines were passing the sound from the switch to the master cylinder. Possible? - Yes.

The switch is located on the top of the FR wheel housing under the carpet panel screwed in horizontally having the connector facing the front of the car. It is located under the metal rectangular plate on the inward-facing rear side of the wheel housing.
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No, 1 light knock every time the pressure in braking system reaches out certain point. If you have the radio on and not focusing on the feeling from the brake pedal, you won't notice it.

As I said in the beginning of the thread, the knock was present with and without engine on. With the engine off the ABS is off. It was no any pulsing type of feelings.

 

As I've read about the suspension system in 355, if the Bilstein ECU learns something is wrong with the system, it shuts down managing the shocks and throwing the amber warning light. Not my case.

Seems as you said unless the switch gets all the time "open" or "close" whatever it should to when the pressure reaching up a certan point it needs to send a signal upon to, the Bilstein ECU could not notice if it's malfunctioning or not? Like if it is never "closing" on the pressure built-up or something.
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I was doing something else with the 355 yesterday and had the front trunk carpet panel off so I was curious and unplugged the brake line pressure switch and the experience was:

• With unplugged sensor the shock absorbers warning light came right up on the engine start and it stayed on. Means the default state of the switch is "closed" and when the pressure is rose to the trigger point, it opens up.
• With the shock absorber adjustment system off the nose diving upon firm braking was just a little more vs. with the system is working. The difference I observed is very little.

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The warning light came up right away on the 1st engine start with the disconnected switch. After that test drive I plugged the switch back in (with the engine off) so on the next engine start the warning light went off.

Don't know if it's a variable resistor based on the switch application. To my understanding the pressure raise of a hydraulic fluid can physically push on a diaphragm which can push the mechanical rod of a switch to open up a circuit, but not sure of how they would construct a variable resistor? I know the BMWs apply the variable resistors to measure the oil change intervals on the principle they expose two contacts of a sensor to the engine oil while having the oil itself is a resisting material. When it gets more dirty, the resistance goes up. In the 355' application, the clarity of the braking fluid remains constant while the pressure is fluctuating. Plus by the name of the device, they call it as a switch. If it will be a variable resistor, they would probably more likely to call it as a sensor.

Also by comparison shocks behavior on braking with the sensor and without the sensor, I was braking differently. Slowly and fast. My observation was the nose diving was about the same degree with the system working which seems when the switch opens up, the Bilstein ECU issues the command to harden the shocks as there is the "window" in the shocks so the gas is not traveling to the next chamber so probably the actuators spin making that window to close and that is pretty much it. Not like the ECU gradually adjusts the softness of the front shocks.

The warning light came off right at the engine start with the disconnected switch with vehicle speed = 0. When the light is up, the shocks system is off regardless car is standing still or moving with acceleration or deceleration.

On a system operating as intended the shocks warning light comes up along with the all the rest of the warning lights on the ignition key turn and then goes off in 1 sec regardless if you depress the brake pedal or not touching it at all. To my understanding this is a check that the bulbs of all of the warning lights are OK.

 

Ian, does your car having the chocks managed by ECU and the anti nose-diving feature? When you brake does your shocks warning light going off? Do you need to press on the brakes upon the engine start to your light to go off?

 

It's just a switch, Ian.

You was right about the resistance. The broken one showing 2 kOhms when it's open. When the pressure rises, it closes.
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I already put back the carpet cover in yesterday. See the description earlier in this thread.

It's where the pointer points down.
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