What Causes Mushy Brake Pedal - E36?

[jdehaan]

Hi everyone! I've been researching causes for why the brake pedal in my E36 Compact track car is so inconsistent. It seems like a common problem, but I can't seem to nail down why this tends to happen for our cars.

For context, the car is an E36 318Ti. It runs stock E36 M3 brakes front and rear (I use an SRS Concepts adapter plate in the rear to use the M3 hardware), with OEM blank rotors and PFC 08 pads all around. I have brass guide pin bushings (get greased every 2 to 3 events), rebuild the calipers annually and I have the new style hard motorsports brake ducts in front. The car had an S52 in it, but was swapped to an S54 this spring. While we were doing that we also swapped in the mk60 ABS system from the donor car. I have an S54 master cylinder (tested it out yesterday and it held pressure well) and the TI booster (with a shim to take up pedal travel). Brake fluid is Brembo HCT64T.


The car is stripped and caged and weighed about 2,650 lbs before the swap (5/8th tank and no driver), about 54% front and 46% rear weight distribution. I haven't gotten a corner balance since the swap, so that's the best data I have. The car is running a stock tune at the moment, but will be going to the dyno soon for a bit more power. I'm already 10+ MPH faster heading into the braking zones after the main straights at the tracks we've run than I was with the S52 and there's still more power to come.


My brakes do a fine job of scrubbing speed (data logger confirms equivalent G forces and MPH reduction as our E36 M3 with the same hardware set up), my problem is in the mushy feel that I know is common among E36 systems. Super inconsistent pedal, almost like there's constantly air in the system despite several bleeds (including bleeding the ABS pump) making it hard to have confidence, modulate and to accurately heel and toe. I found that tapping the pedal before full application reduced the problem quite a bit, so it seems like it's a knock back issue. I've checked on the wheel bearings, which are all tight and there are no fluid leaks to be found.

So my first question is, what causes this issue? Second, what's different about my car that makes the pedal feel so inconsistent when compared to our M3 with comparable hardware? Based on the answers to first two questions, how do I fix it?

[jayjaya29]

Many others will say its the brake booster or master cylinder but I'm convinced the poor pedal feel on E36s is the result of front caliper flex. It gets worse as the pad thickness gets below 50%. If you do a 2 person brake bleed, you can see the caliper flexing when the brake pedal is pumped.

Switch to any other front caliper setup and the pedal firms up. I've tested this by capping the front brake lines off with a bleed valve (to remove air), and the pedal gets nice and firm.

The problem seems to affects some cars more then others.

[pbonsalb]

I did not notice much difference going from M3 calipers with brass guide bushings and braided lines to ST40 and braided lines, but my experience is limited to street driving.

[ScotcH]

Since you have the ABS now, get rid of the booster. That thing sucks, and there is no direct connection to the MC. Just replace it with a bracket, same width as the booster, and run a rod to the MC. SO much better.

[aeronaut]

Since you have the ABS now, get rid of the booster. That thing sucks, and there is no direct connection to the MC. Just replace it with a bracket, same width as the booster, and run a rod to the MC. SO much better.

A slightly fancier version of this is what I'm doing over the winter.
Making a custom bracket to mount Wilwoods "remote M/C mount".
Provides 1.48:1 pedal ratio (x the 4x ratio of the stock pedal) and 2 M/C's.

Massive/Lee also has some good solutions for this.

If you remove the booster and it has the pedal travel sensor, you need to make a mount for that somewhere in the system.

I have the front Porsche/Brembo calipers, and noticed only a small improvement in pedal feel. I've realized (and many others already know) that with any one setup, pedal feel is very dependent on pad thickness. So people replace the OEM calipers and worn pads with new calipers and fresh pads and go wow...when it's really just the fact that the pistons are fully retracted and there's less fluid and play in the system. There are certainly some gains of a fixed caliper over a sliding caliper, but I think the mushy problem is a list of items that all add a little bit of uncertainty in the system. I won't argue that the booster isn't one big factor in the problem, but that's a big decision to make. A little online research, and my estimate is the booster provides 2-2.5x mechanical advantage (~10" booster at about 20-23in of vacuum), so you lose that and live with it or add pedal travel and/or get very high mu pads (I'm doing both).

[olemiss540]

I dont buy that it is caliper flex at all. I went from a stock e36 m3 with absolutely firm brakes to a racecar with wilwood calipers and mush brakes to a racecar with (new) e36 m3 brakes with ok brakes. Guessing that its could be vacuum possibly given the motor swap? Manual brakes take serious effort and I was not a big fan of the setup I tried on track given the amount of pedal effort required but maybe that can be tweaked with different pedal ratios.

I prefer a good working e36 sliding caliper power assisted setup any day of the week. Just have to figure out how to keep it that way!

[jdehaan]

How does one test their booster? I'm not sure I want to ditch it entirely, but can buy that perhaps that's a problem area.

[aeronaut]

Basically, it needs to hold vacuum. You can buy a vacuum pump from harbor freight for a few bucks and rig up a connection to the vacuum booster via the hose that goes to the manifold. Pump down to around 25-30in of vacuum, and see if it holds for a few minutes. Otherwise, check the connection between the hose and the intake manifold, and I'd also check for loose/slack clevis and rods at the brake pedal.

[matthugie]

Bump, has there been any resolution on this? I've had an inconsistent brake pedal the entire time I've owned the car and it has persisted across caliper rebuilds, brass pins in stock brakes, assorted pads and rotors, multiple sets of lines, and entirely new caliper/pad setup (stock to ST40). To avoid the car pulling right under initial application I have to pump the brakes on the straights, but if I pump as early as half way down the straight there is still some mush at the top and the brakes engage on one side first. At this point I'm pretty well used to it, but at tracks like NCM that have some high speed corners where you need to scrub 10-15mph while keeping the car as balanced as possible at entry, not knowing how much brake you're going to get or if the car is going to pull to one side is a legit problem.

[aeronaut]

The challenge is, your symptoms can be caused by several different things. The fact that your car pulls right and pedal is mushy on initial application is a clue (that COULD be pad knock back on the left side, or such.) Are you running stock calipers with brass pins on the rear? I'm not a fan of brass pins. IMO, calipers and pins have to be in near perfect condition for those not to bind.

[matthugie]

The challenge is, your symptoms can be caused by several different things. The fact that your car pulls right and pedal is mushy on initial application is a clue (that COULD be pad knock back on the left side, or such.) Are you running stock calipers with brass pins on the rear? I'm not a fan of brass pins. IMO, calipers and pins have to be in near perfect condition for those not to bind.

I do have the brass guides in the rear and now that you mention it that could very well be the source of the pull. Last weekend at NCM I had to replace the pads on drivers side rear as the inside pad had worn down to the backing plate while the outer had plenty of life left. It's easy enough to swap those out and if that straightens things out then I'll be quite happy, but I'll still be left with it feeling like there's air in the system and an inconsistent initial application.

[aeronaut]

Sticking guides can cause inconsistent initial application and pull to one side.
The extreme uneven pad wear is a good sign you've found the location of your problem.
I hope that's it!

[golgo13]

Bump, has there been any resolution on this? I've had an inconsistent brake pedal the entire time I've owned the car and it has persisted across caliper rebuilds, brass pins in stock brakes, assorted pads and rotors, multiple sets of lines, and entirely new caliper/pad setup (stock to ST40). To avoid the car pulling right under initial application I have to pump the brakes on the straights, but if I pump as early as half way down the straight there is still some mush at the top and the brakes engage on one side first. At this point I'm pretty well used to it, but at tracks like NCM that have some high speed corners where you need to scrub 10-15mph while keeping the car as balanced as possible at entry, not knowing how much brake you're going to get or if the car is going to pull to one side is a legit problem.

I was chasing this problem in 2018 and I solved it by getting the air out of my ABS pump. I found that replacing my M/C and brake booster didn't solve my issue, even after having rebuilt the entire brake system.

https://www.bimmerforums.com/forum/s...Bleeder-Nipple