Brake pedal feel

[simon k]

I've never liked the feel of my brake pedal, my guru reckons my booster is stuffed but I'm not convinced, I spoke to the parts guy at VW today and he said he's only replaced one booster in 30 years of buying bits for them.

The car stops ok, it pulls me up from 170 on the long straight at Winton, but just doesn't feel right, I don't have a lot of confidence in it. The wheels will try to lock and the ABS does it's thing when jumping on the brakes at 50kph.

With the engine running, if I put my foot on the brake the pedal goes 2/3 of the way to the floor, then slowly sinks down to about half and stays there.

Turn the engine off and give the pedal a couple of pumps to evacuate the booster and the pedal is hard, and stays that way, so I don't think it's an air problem unless there's something going on in the ABS unit. I've bled it a few times anyway and I don't think there's air in it.

Last night I pulled the vacuum line off the booster and sucked on it to build pressure then sealed it with my tongue and it didn't lose pressure so I don't think I have a vacuum problem. If there was a vacuum problem I think I'd have a hard pedal, rather than too soft.

I've never driven another Polo GTI (or any other VW for that matter, apart from a late 50's kombi) so I don't know what's normal. We have a Subaru and a Focus as well and their pedal feel is better

I wonder what the brakes are like on other cars.

Thoughts?

[sambb]

my 3 Polo's have all been the same. Every single aspect of what you just outlined inclusing the long pedal at idle. I agree completely with you RE the brakes and feel the same way about them. They are junk in terms of feel. They stop quite well with really good warm tyres on the track but with anything else they go into ABS shuddering at a threshold way earlier than the point you could take the brakes to without it.

[sambb]

The thing is, I don’t find the new car with the Audi TT 225hp brakes any different. That’s 312mm front rotors (versus 288mm for stock Gti) and bigger vented rears too. It has a master cylinder brace fitted and I can honestly say that pedal feel is no different having stepped straight from a Polo without it into one with it. The Bathurst hillclimbs involved some pretty hard sudden stops hauling it down from circa 150km’s maybe and the pedal feel wasn’t different to what I was used to without a brace. Not saying firewall flex isn’t an issue but at the same time the brace didn’t make a noticeable difference to me. The pedal was still sunk way down before it bit and it did so with no feel just like before. The TT brakes don’t feel like they have anymore one off stopping power but will probably stop better for longer on the circuit without overheating – I wont know till I do some circuit stuff- but in terms of feel the pedal is still long, vague and disconnected. I think now I probably will go back to the rear beam off my old car (better geometry, bushing, lighter and stock rears possibly with smaller pistons ) to see if I can find some extra feel.
I think the stock system is either overboosted, or the master cylinder is wrong or both. Is it possible to use a restrictor in a boosters vacuum line to reduce vacuum?
For track stuff and non Bathurst or Huntley hillclimbs I usually pull the ESP fuse. A lot of the intrusion you feel from the ABS system is actually ESP related. With that done obviously you’ll still get that hard braking intrusion but you’ll be free of the same interference in corners. I had wondered about pulling the ABS fuse on the track. I had read somewhere that doing so changes the brake bias towards the rear in some cars – that that is what ABS modules do when they are rendered inactive- but I don’t know RE the Polo’s. That obviously wouldn’t do anything re pedal feel though but will definitely stop ABS intrusion in hard stops and would null the ESP at the same time too I assume. Worth a try?

[simon k]

Thanks Sam, I appreciate it.. sounds like it's just a Polo thing. We're in Melbourne over Easter so I might try to find one for sale to try to test drive.

Is pulling the ESP fuse.different to the button on the dash?

[genebaby]

If it helps we have a 2019 AW Polo, just the standard one, the break feels great in every day driving. Wouldn't change a thing.

[simon k]

If it helps we have a 2019 AW Polo, just the standard one, the break feels great in every day driving. Wouldn't change a thing.

Thanks, but doesn't really help, 13 years and no interchangeable parts....

[Sydneykid]

Obviously you guys will have bled the hell out of them so unlikely to be air in the system. But ABS systems are notorious for trapping air that is a bitch to get out, pressure bleeding is the only guaranteed method.

Fire wall flex is unlikely due to the design of the hardware, and Sam's experience seems to confirm that.

Boosters are often an issue on turbo charged cars as they work best on vacuum and not so well on boost. Check if there is a boost versus vacuum difference in the brake pedal feel.

Lastly master cylinder size, it was common practise to "under size" the master cylinder to give more progression in the pedal effort with ABS systems 10+ years ago. This results in a "long pedal" that can be vague, especially approaching the traction limit. Tyres are almost always the limiting factor in braking, it's very unusual to find a road car in the last 20 years that can't generate 1g under brakes, that's why ABS systems were invented, to help overcome tyre deficiencies. Upgraded braking systems are for performance consistency and longevity. Hence often don't give an instantaneous improved in braking G forces, unless the tyres and suspension are upgraded to cope with the increased braking performance.

For combo road and track cars I have in the past increased the master cylinder bore size to give a more solid pedal feel. Of course this means more effort required in pushing the pedal but with a boosted system that's hardly an issue. Knowledge of the bore sizes of compatible master cylinders might be of use, if you can find one with similar configuration with a larger bore then that will benefit the pedal feel.

Cheers
Gary

[genebaby]

I hear you but you shouldn't have such a bad brake feel. I'm not happy with a car if the brakes don't perform and feel good.

It should be fixable, you don't have a 50's car.

Fluid, lines and pads are what you may need.

[simon k]

Excellent, thanks all

Plan of attack



1. Proper bleed, I've booked in to VW for a diagnostic check to get them to see if there's anything funny going on. I'll see if I can get them to hook it up to their bleeder machine. I did plug my mates OBD test computer in to it and ran through the brake cycle test and it did everything it was meant to. Can I bleed with VAGCOM at home? I know someone with the proper cable & licensed software

2. Test drive another Polo GTI to compare first hand

3. Look at bigger bore MC (I do like this idea)

[simon k]

It should be fixable, you don't have a 50's car.

The brake feel on my 1970 car with 50's technology is absolutely beautiful, I wish the Polo was as simple!