Brake Pedal Feel Issue

[MasterKwan]

As someone pointed out, at least on my car it's dependent on pad wear. New pads give me good pedal feel. The more they wear, the worse the pedal feels until I replace the pads and everything is good again.

I've done the booster, I've done the MC. Brass and rubber, nothing makes a difference. I've been wondering lately if it's something in the ABS unit leaking down. The pad wear thing suggests though that it's related to how far the pistons are sticking out of the calipers.

[Brent 930]

Barring you don't have any air in the system, booster is good (I no longer believe the booster is over boosted), MC is good, brake lines are good (not soft), there are only a few things it can be.

-Large front single caliper pistons
-Mushy brake pads (race pads will feel better apples to apples)
-Un matched pad to rotor surface (new rotors and old pads or vice versa)
-Tapered brake pads
-Uneven spec'd rotors, which could be OE or worse quality.

Other technical ideas are; solid pedals vs. OE rubber, thin hard sole shoes (driving shoes) and thin socks vs. say running shoes and thick socks.

Technically a restrictor for the booster is just a band aid, however it will do one nice thing for you. It will help reduce the initial first pump of the brake pedal.

[VengeanceM3]

As someone pointed out, at least on my car it's dependent on pad wear. New pads give me good pedal feel. The more they wear, the worse the pedal feels until I replace the pads and everything is good again.

I've done the booster, I've done the MC. Brass and rubber, nothing makes a difference. I've been wondering lately if it's something in the ABS unit leaking down. The pad wear thing suggests though that it's related to how far the pistons are sticking out of the calipers.

After replacing pretty much everything I've found this to be the case as well. It kinda sucks as when my pads get close to ~50% the feel starts to really be awful - hugely noticeable change from new pads (though I still think even on brand new pads the feel is not spectacular). I'm thinking about making a set of shims out of old backing plates, as was recommended to me by someone else. Any issue with essentially having a second "loose" backing plate in there between the piston and the real pad...other than the terrible noise I'm sure itll result in?

[benaj]

Both my wife and I use stock E36M3 brakes on our track cars and have been using these shims for years to compensate for worn pads. I initially started to do it to keep the brake pedal height the same for heel-toe consistency as the pads wear down, but now add them in even when pads are only mildly worn to keep the brake pad "stack height" the same as new pads just for pedal feel, not just pedal height.

To make them you just chisel off all friction pad material from an old set of pads and then drill a hole in the center to compensate for the rattle clip nipple on the good pad set. You can then add spacers to either inside or outside pad (or one on each side if needed). The secret is to know total thickness of a new set of pads and then use a combination of spacers to get your used set as close as possible to new thickness. BTW is it possible to to add too much spacer on the inside pad. Once or twice I have installed a spacer on that inside pad only to find that although I could get the caliper reinstalled with the spacer in, the pad was then so tight on the rotor once everything was installed that the rotor would bind and not rotate. I would then have to pull it back out until after the next session when the pad was a little thinner.

Haven't noticed any extra noises when these brake spacers are in place, but then again my car makes so many different kinds of loud obnoxious noises that I wouldn't hear the brake pads even if I took my ear plugs out when I drove it.

good luck.

[LVP]

Weird - stacking pads. I can't believe that this issue plagues so many cars.

I tried stomping the pedal last night and even with it off, taking up all the slop in the system moves the pedal too far down. So boosted or not, car on or off, the pedal travel seems the same.

The pads I installed on the weekend (carbotech xp16's and xp12's, F/R) really pull her down quite nicely. Can't wait to test on the track.

A friend of mine has a 1996 318ti with a solid pedal and good height. We're comparing systems and specs. I'll post any findings to this thread.

Cheers.

[osborni]

Both my wife and I use stock E36M3 brakes on our track cars and have been using these shims for years to compensate for worn pads. I initially started to do it to keep the brake pedal height the same for heel-toe consistency as the pads wear down, but now add them in even when pads are only mildly worn to keep the brake pad "stack height" the same as new pads just for pedal feel, not just pedal height.

To make them you just chisel off all friction pad material from an old set of pads and then drill a hole in the center to compensate for the rattle clip nipple on the good pad set. You can then add spacers to either inside or outside pad (or one on each side if needed). The secret is to know total thickness of a new set of pads and then use a combination of spacers to get your used set as close as possible to new thickness. BTW is it possible to to add too much spacer on the inside pad. Once or twice I have installed a spacer on that inside pad only to find that although I could get the caliper reinstalled with the spacer in, the pad was then so tight on the rotor once everything was installed that the rotor would bind and not rotate. I would then have to pull it back out until after the next session when the pad was a little thinner.

Haven't noticed any extra noises when these brake spacers are in place, but then again my car makes so many different kinds of loud obnoxious noises that I wouldn't hear the brake pads even if I took my ear plugs out when I drove it.

good luck.

Ben, just throw on a set of Wilwoods. Pads are cheaper too.

[TOOLEAN]

I run shims as well, no issues here.

[osborni]

I do too (but on fixed calipers) and for the same reason - seams to give better feel and firmness.

[Brent 930]

It seems new pads or using shims for worn pads is a direct relation to the big front piston. However I have yet to hear about a mushy brake pedal from the E46 owners. As you know the E46 M3 also uses a big piston and has the same size front pad as the E36 M3.

There doesn't seem to be just one thing that causes the mushy pedal.

[jmsanders78]

I have an E46 328i and feel mushy pedal when the pads get some wear. If I recall, the part number for the front calipers on my car are the same as for the E36 325s so if the calipers are the problem then some E46s should have the same general problem. I switched from Hawk to PF on advice from someone else that didn't like the Hawks and had had a similar experience... So far so good, but they aren't very worn yet.

[LivesNearCostco]

OK everyone, in spite of not having an E36 or any kind of M3, nor knowing much about how brake boosters work, and not knowing if my car has mushy brake pedal feel or not, I have read almost this whole thread and put forth two theories. Tell me if either one makes sense.

• The amount of boost increases as you push the pedal down further. Something about the E36 M3 brake booster increases the vacuum assist as you push the pedal further. With fresh pads you get normal boost, but once pads wear beyond 50%, you get extra boost. Maybe an extra port to atmospheric pressure opens up on the booster, or maybe the amount of vacuum applied to the whatever-it-is (diaphragm?) in the middle of the booster somehow increases as it moves closer to the master cylinder.
• The fluid in the calipers starts to boil as the pads get thinner because thinner pads provide less insulation against heat.

2nd theory seems unlikely because everyone says the cars still stop fine even with the mushy pedal feeling. Going with 1st theory, someone should take apart an E36 M3 booster and find out if it has more than one port to let in atmospheric pressure on the driver side or more than one path to let in vacuum on the MC side, and then block off that extra port. Just a wild and crazy theory from Costco-land. (For whatever reason, Costco sells tires, cars, wheels, car batteries, motor oil, shop towels, and sometimes floor jacks, but they never sell brake pads.)

[LVP]

I have put feelers out to see if anyone up here in Southern Ontario wants to donate one to the cause of getting some answers. My friend is a mechanical engineer, so at the very least we'll get a better understanding of how it works, but more importantly, where does the slop exist and how can we eliminate it.

I'm going to make up some spacers from some dead pads I have. Heading to Cayuga this weekend for a test.

Keep the ideas coming.

Cheers!

[Brent 930]

Costco, Ummm no.

-The brake booster doesn't increase vacuum.

-Brake pad wear has nothing to do with extra boost assistance. More fluid is needed as the pads wear.

-There isn't any port that opens up.

-The only vacuum the booster sees is what the motor is pulling.

However taking a booster apart to see the design inside might be cool.

[LivesNearCostco]

Come on, don't discount my theory just because I don't know any physics!

I didn't mean the motor creates more vacuum as the pads wear. I meant that either more of that vacuum got applied to the MC side of the booster or more atmospheric pressure was applied to the pedal side of the booster as the pedal was depressed further. Intake vacuum applied equally to both sides of diaphragm. Brake pedal opens valve to let atmospheric pressure into firewall side of booster, pushing diaphragm towards MC. If valve opens more to let air in faster, or does it not matter because the pedal side of the booster is instantly at 1 atm. so opening the valve more won't make any difference? If the diaphragm surface area increases as it moves, would that increase brake boost?

But I see now my theory doesn't hold water (or hydraulic fluid). As the pads wear more, more fluid goes down the lines and the pedal drop should remain the same. Unless, as others mentioned here, worn pads have more taper or more knockback. I must have smoked too many Tyvek wristbands.

Maybe as the pads wear and the piston sticks further out and tends to sit cockeyed in the bore. Or the slide pins let the caliper flex more and/or get out of alignment with the bracket, which then causes pad taper. But uhhh.. those theories were already covered in this thread, right?

[osborni]

As the pad wears down, the MC pumps more fluid into the caliper, so the brake pedal should, in theory, be at the same height as the pad wears. Each application should, in theory, displace the same fluid volume given the same pedal application force (plus a wee bit more to make up pad wear for each application).

BUT, what changes is how far the piston sticks out and how the pad interfaces with the rotor and how the piston interfaces with the pad. I suspect that on a floating caliper there is more knockback as the pad wears down as well as more piston cocking as it sticks out further. The piston is rather large in diameter and when there is not a lot is left in the caliper body it can cock rather easily.

I've also never had a pad on a floating caliper that wore out evenly, even with brass bushings. They either tapered radially or axially or both. So as it wears, the piston needs to force the tapered pad onto the face of the rotor with all the associated movement and make-up.

I also suspect that parts inside the booster start to wear and it doesn't perform consistently over it's life with different stroke lengths. But not sure on that one as I've never seen inside a booster. At any rate, most of the issue is probably at the piston/pad/rotor interfaces. There really isn't a solution unless new pads are always used, pads rotated after every session or a fixed caliper is installed.

[n24tg]

I'm curious as to whether an E46 330 or E46 M3 master cylinder would fix the problem.

[MasterKwan]

I don't think it has anything to do with the MC so, I'd say no.

As the pad wears down, the MC pumps more fluid into the caliper, so the brake pedal should, in theory, be at the same height as the pad wears. Each application should, in theory, displace the same fluid volume given the same pedal application force (plus a wee bit more to make up pad wear for each application).

This is how I think of it too. In fact, I've had numerous non-BMW track cars with single pot slidy calipers and they wore evenly and acted like you expect. Fluid from the MC kept the pedal at the same height. So, I think this issue is related to how the calipers are mounted or the size of the pistons. The brakes act as if they have more knockback, the more the pad is worn. So, the clearance between the pad and the rotor opens up as the pad wears. That would explain the lower pedal, it takes more fluid to close the gap between the pad and rotor.

[osborni]

Yeah - I think it's simply that stuff gets more loose the more the pad wears. It takes more pedal from 1st pad contact to full pad contact to the rotor.

With fixed calipers the pedal feel is the same regardless of pad thickness.

[LVP]

So, an update. To recap what I have now done on my 1998 328i:

Stock calipers, fully rebuilt, brass guides
Stainless lines
ATE Blue/Yellow fluid (alternate to know it's purged)
Carbotech pads (XP16 fr@100% / XP10 rr@50%)
Cross drilled rotors
Beld, bled, bled - pedal, pressure, ABS valves cycled, etc.
M3 Master
No leaks
Adjustable valve installed on the vacuum line to the booster
Pad shims on rear

I took it to Cayuga on the weekend and the pedal still sucked as far as travel goes. I did not have the shims in, but it was fully bled and fresh fluid. Since the track day, I pumped a different colour, fresh fluid through it and inserted a set of pad shims on the rear. Still no difference. The vacuum line valve also does nothing remarkable as far as travel goes and just restricts how quickly the vacuum rebuilds after initial press. Close the valve off and you have complete manual brakes. As for pad knock back, all this can be experienced with the car on stands and no rotors turning, car on or off.

So, I'm back to finding a booster to rip apart and see what's going on inside to contribute to the travel.

And for those interested - my track pads rattled about anyways, but with the extra spacer plates, the car sounds like a bag of loose bolts. The nice thing about it though is you hear none of it on the track

Cheers.

[MasterKwan]

Your results are much worse than mine.

How was it set to manual brakes? How much harder did you have to step on the pedal?

I thought the vacuum line valve was a check valve that would keep vacuum in the booster even when you're wide open throttle so, you always have boost even after a period when the engine itself had no vacuum.

[Brent 930]

I thought the vacuum line valve was a check valve that would keep vacuum in the booster even when you're wide open throttle so, you always have boost even after a period when the engine itself had no vacuum.

That is correct, plus it helps for FI cars.

LVP, your next step should be to consider a nice front BBK.

[LVP]

The check valve in the line does that. I put a hand valve in the line that allowed me to vary the feed to the booster. With the valve almost closed and after some period, your first pedal press felt boosted. If you hit the pedal in rapid succession after it, the line is throttled so much you can't build the required vacuum in time and it's like having full manual brakes. It gives a false sense of hope in that the pedal feels stiffer/higher, but really, when you measure it, the travel is the same. Manual brakes on the street are not comfortable. It requires a lot of leg effort and more importantly, you need to adjust your stop distance mental calculations.

So, now I have a fancy valve in my vacuum boost line that looks cool when I open the hood and tell people that have no idea what I'm talking about that I can adjust my brakes just like my suspension. If only it actually worked.......

On a plus side, I was able to pull a gap on an Audi R8 V10 on the weekend. Small victory. He still got to drive home in his R8 while I swapped tires and shed a tear for my crappy brake pedal. That balanced things out in the end.

Cheers.

[Brent 930]

A bleeder in the vacuum isn't the answer, but you already know that.

IF you have faith that the booster is allowing the brake pedal to travel too far (thus there is a point where the brakes are full on) then put in a brake stop.

[LVP]

I don't have that much faith given how much I've to chase this out. Based on some cutaways of boosters i"ve found online, I know there are certain things that need to happen in a certain order, but I'm hoping that I can install something in the booster to take up the slack and engage the pistons/rods to one another sooner. Too much to speculatae on until I get one in my hands to rip apart. Once I do, I'll post pics, findings, theories and possible solutions. I know there are a lot of people that would like to know these details.

[Brent 930]

LVP, it's a plausable idea to check out. It's possible your brake booster diaphrams are worn out inside causing some of the mushy pedal, and a new booster would help some. However you will never win the battle of the big front sliding caliper design and piston.