Help: Mysterious Soft Brake Pedal

[3002 tii]

Would be great if anyone can provide insight on this mysterious soft brake pedal I've been experiencing during these past 2 weeks. First occurred when entering pre-grid at NJMP. As I pull up to my spot and try to slow down the pedal went all the way to floor. Pumped the brake and everything felt 'normal' but needless to say I didn't set any track records during that session.

Thought it was an isolated event but it happened again 2x more... once as I pull up to a red light and once as I was backing into my parking spot. For what it's worth, pads and rotors have tons of life left and I did a full flush 2 weeks ago, followed with 4 corner bleed this past weekend.

Could it be the master cylinder failing? Someone on BFC mentioned checking the wheel bearings as that could cause knock back. Haven't had a chance to pull the wheels off so I'll do that next.

[Vanne]

Wow.. man, i had this exact same thing when i took my car in for my BBK upgrade. As i drove into the garage, pedal went as you said all the way to the floor, scared the shit out of me, then re-pumped and was back to normal.

I instructed my mechanics to check the whole system during the BBK install, after telling them the story. They found nothing and were unable to reproduce said event.. at all.. period.. I instructed them to pull the master cylinder and refresh all the seals. Ive since driven the car after the new BBK install, new high temp fluid, for probably 500 KMS, with zero events..

The seals in the master cylinder/actuator were also found to be in great shape...

Fricken crazy thing eh??

edit: wheel bearings to cause knock back?? man those would have to be some seriously rooted wheel bearings.. and thats certainly not it for me, new bearings front and back less than 10K kilometres..

[StickMon]

Haven't experienced this with the Zed, yet, but on all 3 of my Suburbans it was master cylinder.

[3002 tii]

Originally Posted by Vanne

Wow.. man, i had this exact same thing when i took my car in for my BBK upgrade. As i drove into the garage, pedal went as you said all the way to the floor, scared the shit out of me, then re-pumped and was back to normal.

I instructed my mechanics to check the whole system during the BBK install, after telling them the story. They found nothing and were unable to reproduce said event.. at all.. period.. I instructed them to pull the master cylinder and refresh all the seals. Ive since driven the car after the new BBK install, new high temp fluid, for probably 500 KMS, with zero events..

The seals in the master cylinder/actuator were also found to be in great shape...

Fricken crazy thing eh??

edit: wheel bearings to cause knock back?? man those would have to be some seriously rooted wheel bearings.. and thats certainly not it for me, new bearings front and back less than 10K kilometres..

I hope it's the master cylinder as that's cheaper to replace (reputable oem ones are 1/10th the price of BMW Genuine).

Yea I didn't realize this but apparently if the rotor deflects enough laterally it can cause the pistons to move in so when you push the pedal most of the travel is used to push the piston back to the rotor.

[3002 tii]

Originally Posted by StickMon

Haven't experienced this with the Zed, yet, but on all 3 of my Suburbans it was master cylinder.

Was it the just the seals or the cylinder itself?

[StickMon]

I think the seals are the only part that has any probability of failure, but I've never actually examined or replaced them, so I don't know 100%.
On something like a Suburban, you don't replace the seals, you just run up to AutoZone and get a rebuilt.

[inTgr8r]

Originally Posted by 3002 tii

I hope it's the master cylinder as that's cheaper to replace (reputable oem ones are 1/10th the price of BMW Genuine).

Yea I didn't realize this but apparently if the rotor deflects enough laterally it can cause the pistons to move in so when you push the pedal most of the travel is used to push the piston back to the rotor.

Knock back is usually not an issue on floating calipers.

[sipman]

Take a close look at the ears of the brake pads themselves. Since we push back the pistons almost all the way into the slave cylinder when we swap from street to race compound pads, there is sometimes more space than needed especially if the race pads have some wear on them.
If you don't pump your pedal before reinstalling your wheel, you may miss the time where your pad falls over onto it's side and becomes wedged against the rotor. The bottom ear is now bent over as top of the pad is leaning against the rotor. The metal backing plate effectively becomes a spring and returns the piston to it's original position, but hasn't advanced the pad towards the rotor. That'll cause the soft pedal with a long travel.
I know this problem so well because I went through a set of R-4s because of this. Now I pump the pedal and ensure that the piston and pads are tight against the rotor with no binding.
Hope this helps!

[dc_wright]

Originally Posted by 3002 tii

I hope it's the master cylinder as that's cheaper to replace (reputable oem ones are 1/10th the price of BMW Genuine).

Yea I didn't realize this but apparently if the rotor deflects enough laterally it can cause the pistons to move in so when you push the pedal most of the travel is used to push the piston back to the rotor.

I had this happening with the Ferodo DS2500 pads I had on the front. The spring clips on the inner pads didn't lock into the pistons and the pads were pushing out against the rotor and moving the pistons back until there was clearance between the pads and the rotors. Intermittently I'd get 3/4 or so of pedal travel with NO braking at all, and then one pump and I'd have full pedal height back. It was intermittent because when I'd step on the brakes it would reseat the pads in the piston, and sometimes they'd stay in place for a bit, and sometimes they wouldn't. VERY disconcerting driving like this to point where every stop was a subconscious double pump.
I had replaced the rotors when I installed the pads, and an out of line rotor can definitely give you kick back, and I somewhat suspected that first, but it didn't explain why it was intermittent. Same with a bad wheel bearing.
The way I finally found it was the pads was driving the car up and down our street and lightly testing if I had full pedal. When I got the start of feeling like it was going to be a "long" pedal I used the e brake for braking, got the car back into the garage, on the jack stands, pulled the wheels and visually inspected each caliper to see where the pads were sitting and how much if any gap had developed between pads and rotors. I had about 1/16" on each of the front calipers and by experimentation I could see the pads weren't locked into the pistons. Replaced the Ferodos whith some EBC Yellows (because I could get the next day Amazon Prime and I had a track day 3 days from then), and problem gone.
1/16" doesn't seem like a lot but when you think about when you replace brake pads and have pushed the pistons all the way back into the caliper you have maybe 1/8" of clearance when you put the caliper back over the rotor. It usually takes 2 or 3 full pedal travel pumps to get the calipers refilled and the pads against the rotors so getting almost to the floor on one pump with 1/16" isn't unreasonable.
3002tii, I think I'd be looking to see if you've had a pad retainer spring break on one of the inner pads which would let a pad drift out of the piston and give you "knock back" or push back, particularly if what you felt is an initial long pedal travel like you had no brakes and then firm braking after a pump. If you had some brake feel but it was spongy until you pumped and then it got firm, I'd be looking at master cylinder.

[Vanne]

Intresting.. with me there was no brake feel at all, just like you guys said, when your pumping to push the pad to the rotor.. having my fingers crossed that what it was. Either way, since then i now have a completely new brake system.

[3002 tii]

Originally Posted by dc_wright

I had this happening with the Ferodo DS2500 pads I had on the front. The spring clips on the inner pads didn't lock into the pistons and the pads were pushing out against the rotor and moving the pistons back until there was clearance between the pads and the rotors. Intermittently I'd get 3/4 or so of pedal travel with NO braking at all, and then one pump and I'd have full pedal height back. It was intermittent because when I'd step on the brakes it would reseat the pads in the piston, and sometimes they'd stay in place for a bit, and sometimes they wouldn't. VERY disconcerting driving like this to point where every stop was a subconscious double pump.
I had replaced the rotors when I installed the pads, and an out of line rotor can definitely give you kick back, and I somewhat suspected that first, but it didn't explain why it was intermittent. Same with a bad wheel bearing.
The way I finally found it was the pads was driving the car up and down our street and lightly testing if I had full pedal. When I got the start of feeling like it was going to be a "long" pedal I used the e brake for braking, got the car back into the garage, on the jack stands, pulled the wheels and visually inspected each caliper to see where the pads were sitting and how much if any gap had developed between pads and rotors. I had about 1/16" on each of the front calipers and by experimentation I could see the pads weren't locked into the pistons. Replaced the Ferodos whith some EBC Yellows (because I could get the next day Amazon Prime and I had a track day 3 days from then), and problem gone.
1/16" doesn't seem like a lot but when you think about when you replace brake pads and have pushed the pistons all the way back into the caliper you have maybe 1/8" of clearance when you put the caliper back over the rotor. It usually takes 2 or 3 full pedal travel pumps to get the calipers refilled and the pads against the rotors so getting almost to the floor on one pump with 1/16" isn't unreasonable.
3002tii, I think I'd be looking to see if you've had a pad retainer spring break on one of the inner pads which would let a pad drift out of the piston and give you "knock back" or push back, particularly if what you felt is an initial long pedal travel like you had no brakes and then firm braking after a pump. If you had some brake feel but it was spongy until you pumped and then it got firm, I'd be looking at master cylinder.

Everything you said makes total sense especially about the intermittent aspect. But say it is the spring pad clip, then there's really nothing I can do short of replacing the pad? That would suck given these pads have a lot of life left to them.

[dc_wright]

Originally Posted by 3002 tii

Everything you said makes total sense especially about the intermittent aspect. But say it is the spring pad clip, then there's really nothing I can do short of replacing the pad? That would suck given these pads have a lot of life left to them.

Assuming it's a broken clip, if it's the vertical leg you might be able to bend the two lateral legs out for some more tension from them and try that. If it's one of the lateral legs of the clip then I think you're done at that point.
It does certainly suck! The Ferodo's were the most expensive brake pads I've ever bought to date. I've tried reforming the pad clip to try to get them to hold in place, but no success. They were special order for the 3.0si so it was a no refund, no return purchase. At best I might be able to wrangle another set out of them with some kind of proration charge to me, but at this point I'm done with Ferodo's regardless so moving on. Lesson learned.
Side note: I did leave them in the rear, and they were somewhat short lived, so even if I hadn't had the spring clip issue I think I would have been back where I am with the Hawk pads again.

[3002 tii]

Originally Posted by dc_wright

Assuming it's a broken clip, if it's the vertical leg you might be able to bend the two lateral legs out for some more tension from them and try that. If it's one of the lateral legs of the clip then I think you're done at that point.
It does certainly suck! The Ferodo's were the most expensive brake pads I've ever bought to date. I've tried reforming the pad clip to try to get them to hold in place, but no success. They were special order for the 3.0si so it was a no refund, no return purchase. At best I might be able to wrangle another set out of them with some kind of proration charge to me, but at this point I'm done with Ferodo's regardless so moving on. Lesson learned.
Side note: I did leave them in the rear, and they were somewhat short lived, so even if I hadn't had the spring clip issue I think I would have been back where I am with the Hawk pads again.

Yea I'm running pfc 08's and they're not cheap, and have served me well over the years. But not knowing if I have brakes really sucks. On the flip side I've actually been practicing left foot braking to test the brakes on any fast straights lol.

Now I have more reason to try winning Hawk contingency at my next event.

[F360C]

Originally Posted by 3002 tii

Yea I'm running pfc 08's and they're not cheap, and have served me well over the years. But not knowing if I have brakes really sucks. On the flip side I've actually been practicing left foot braking to test the brakes on any fast straights lol.

Now I have more reason to try winning Hawk contingency at my next event.

Are you on stock calipers or aftermarket? Do you have after market sway bars?

[3002 tii]

Originally Posted by F360C

Are you on stock calipers or aftermarket? Do you have after market sway bars?

Stock brakes and sways

[dc_wright]

Originally Posted by 3002 tii

Yea I'm running pfc 08's and they're not cheap, and have served me well over the years. But not knowing if I have brakes really sucks. On the flip side I've actually been practicing left foot braking to test the brakes on any fast straights lol.

Now I have more reason to try winning Hawk contingency at my next event.

The instructor at my first PDX several years ago encouraged me to do this, and I've continued. I left foot brake for autocross so slipping my foot over to give a quick test comes pretty naturally. I do it at roughly the 700-800 foot point before the turn depending on the track and what kind of run off area (if any) there is at the turn.

I know several people that run the PFC 08's on their track cars and really like them. I haven't gotten much feedback on autocross use though...

[3002 tii]

Originally Posted by dc_wright

Assuming it's a broken clip, if it's the vertical leg you might be able to bend the two lateral legs out for some more tension from them and try that. If it's one of the lateral legs of the clip then I think you're done at that point.
It does certainly suck! The Ferodo's were the most expensive brake pads I've ever bought to date. I've tried reforming the pad clip to try to get them to hold in place, but no success. They were special order for the 3.0si so it was a no refund, no return purchase. At best I might be able to wrangle another set out of them with some kind of proration charge to me, but at this point I'm done with Ferodo's regardless so moving on. Lesson learned.
Side note: I did leave them in the rear, and they were somewhat short lived, so even if I hadn't had the spring clip issue I think I would have been back where I am with the Hawk pads again.

Ruled out broken clips and front bearing seem fine. Don't think it's the master since the soft pedal is intermittent. Bled the lines a 3rd time, pedal feels stiffer so let's hope that fixed it.

[Vanne]

fingers crossed.