Brake pedal feel

[simon k]

yeah..... so Winton was ok-ish, I'm definitely still not happy with the brakes, I don't think the change of pads or pedal brace made a huge difference. What did happen though was half way through the day the pedal just went to sh*t, I suspect the fluid boiled, I bled it the day after and it came out darker than it went in. I also checked the brace and one of the locknuts had backed itself off and the bolt wasn't as tight as it was. I've done it up and maybe the pedal is better, but nothing to write home about

I did a rally school day once and they talked about road braking (the called it "taxi braking") v's race car braking. On the road, people usually start with light pressure on the brake pedal and increase pressure the closer they get to the stopping point. For example, coming up to a give way sign you brake gently as you approach, looking at the intersection (hoping to glide through) and checking for cars, as you get closer you brake more while you keep checking, then a car approaches and you brake harder and come to a stop. On a racetrack you do the opposite, coming in to a braking zone you brake as hard as you can, then release pressure as the car approaches turn-in speed, then you start to turn in and release the brakes. The two methods are opposite...

Driving the Polo on the road with a vague brake pedal is OK, push the pedal gently until the car starts to slow down, then push a bit harder as you need to brake more and come to a stop.

Pressing the brake pedal a small amount does slow the car down, but not much compared to the other cars we have. It's like you push the pedal and watch the speedo needle go to the left, rather than push the pedal and feel negative G's. My daughters Focus feels the way I want the Polo to feel. When you touch the brake pedal on that car it grabs straight away... even getting in my 50 year old mini I feel like the brakes are actually doing something when I push the pedal

Braking hard at the end of a straight feels like a gamble, I know that the car is going to pull up, but I don't know how hard to push the pedal to make it slow down the way I want.. I mash the pedal and regulate once the car reacts

I'm a kinesthetic person, I sense and feel rather than analyse so I can't describe it the way Sam does

[sambb]

I'm pulling fuse 23 tonight to kill the ABS. I want that pump/module unit acting as nothing but a manifold for a while to see what effect its having.
This is what I dont understand. There's a finite amount of fluid in the system once the pedal is pushed and the master passes the reservoir port. The pads can only move so far. If a good non-abs push of the brakes can haul you down from from 160kph and the pedal only goes halfway, how is that a crash stop with full abs at a slow speed will see the pedal go to the floor. Where's the fluid going? The brake pads didnt move further than the first instance. The fluid didnt fall on the ground. The master cylinder didnt suddenly blow a seal. To me there has to be some dynamic internal accumulator (picture a balloon expanding or contracting if a solenoid gives fluid a pathway there?) inside the abs module that might even be active all the time, not just in a full abs intervention. Or could it be that a bypass circuit is opened up in the module that actually pulls fluid from out in front of the master cylinder when the abs becomes active so that you cant add any additional force with your foot to the pre programmed abs routine thats being pressured by its pump. Dunno. Why else would the pedal literally become redundant in an ABS stop. I'm thinking they dont want you having any input at that stage to how the braking is performed. Where I'm going with all this is that it really could be some sort of active line pressure management thats governed by the ABS module, which is making our pedals feel so long, vague and variable. If its a dynamic system thats active all the time to varying degrees ie not just on when a full ABS stop happens, then that could be what is killing the pedal feel, because you are never actually feeling feedback through the pedal of the fluid pressure directly against the pads - its damped and tampered with through the module.
I've seen it written here and there that the pedal gets better immediately after a crash stop. Maybe after said accumulator or bypass circuit is utilised the system then evacuates it again which restores some pedal feel. Well that would be the right time to pull the ABS fuse in my opinion.
Simon does your daughters car have ABS? If you do a crash stop does the pedal head for the floor or stay up where it normally should be?

[simon k]

Here's a nice simple explanation of how the ABS pump works.. Understanding ABS Modulator Problems | Know Your Parts

Not sure about the Focus, it definitely has ABS (2015 model) but I haven't done an ABS stop with it. The Subaru maintains the pedal height when doing an ABS stop, the pedal just vibrates/pulses underfoot, it doesn't fall away.

[Sydneykid]

I'm no expert on ABS systems, but from long time ago training on them I recall that they commonly have 2 valves, per wheel. The inlet valve (sometimes called an isolation valve) isolates its brake from the ABS pump, accumulator and master cylinder. The outlet valve (sometimes called a dump valve) allows the brake pressure to be released, for example when a locking of that wheel is detected. When you brake the inlet valves are open, the outlet valves are closed and the brakes operate as "normal", it's only when the sensors detect lock up that the valves work together to release the pressure and unlock the wheel.

Maybe if the outlet valve is leaking (worn seat, sticky solenoid etc), when it should be closed, that causes the spongy pedal.

I found the the schematic helpful (same link)
https://www.knowyourparts.com/technical-resources/brakes-and-brake-components/understanding-abs-modulator-problems/

Cheers
Gary

[sambb]

Ok pulling the cabin fuse 23 kills ABS/ESP/traction but also takes out the power steering and speedo. The way to do it is to pull fuse 6, the normal blade fuse ( I think 25 amp) in the engine bay. It'll kill ABS without affecting the power steering or the speedo. Problem is I can confirm:
1. it doesnt improve the pedal
2. The rear biasing is very real to the extent that if you were to even do an abrupt stop because you were a bit caught out by a red light it will lock the rears. Braking hard in a straight line to get the car to pitch and its literally like a hand brakey. Would be sweet for autoX or dirt khanacross though!

So there's not any ABS module isolation solution that I can see RE pedal feel improvement.

The ESP killing fuse is cabin fuse panel, furthest right column, 2nd down from the top 5A. It'll take your dash clock with it and kill the chime to tell you you've left your lights on. But ESP is rendered inactive and the car does feel very different when hustled through corners. Maybe that'll help keep ABS from jumping in to help when any brake application is happening during cornering eg trail braking into a corner and the inside rear is in the air.

Its got to be the MC or booster

[Sydneykid]

If the outlet valves/s are leaking then pulling the fuse won't help.

Cheers
Gary