Very spongy brakes....

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Plastic_Badger

Member
I've owned my Z (3.0Si Auto roadster) for about 3 weeks now and love everything about it but one thing....... the brakes!

After a couple of big stops from motorway speeds they seem to go very spongy and really don't inspire much confidence. I've come from a E92 3 Series Coupe and the brakes on that were far superior.

The discs and pads are all in good condition so they aren't defective but compared to other "fast" cars I've had (Golf GTi Turbo, Audi TT, S2000, even the 106 GTi) they have to be the least confidence inspiring.

Maybe it's because it's quicker and so I'm having to haul it down from higher speeds or it's due to being an Auto and lack of engine braking.......

Has anyone else found the same??
 
I have had similar problems, and a brake fluid change hasn't helped at all. Let me know how you get on!

Sent from my GT-I9100 using Tapatalk 2
 
Not the same braking system but i had the same feeling on my M after the garage did a disc/pads/fluid replacement for me. Kept bugging them to see what was wrong and they said nothing was wrong. After a while i got so fed up that i bled them myself with a Motive PowerBleeder and lo and behold the sponginess disappeared! Note that i bleed my brake fluid at least once a year.
 
Mine went from fine to spongy after a BMW dealer visit for a scheduled brake fluid change (for the stamp only).

Picked the car up, went home, and re-bled the system so it was back to normal. Took about 30 minutes to get all the air out by going from caliper to caliper.
 
Is brake bleeding easy for mechanical-noobs?

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Yes it is an easy task, as its a tricky one person job best to get one of the many brake bleeding kits that are availiable
 
dan425 said:
Is brake bleeding easy for mechanical-noobs?
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If you can do an oil change, you can do a brake bleed.

If you're by yourself you may need a power-bleeder kit which attaches to your brake fluid reservoir, and when pressurised pushes fluid through the system.
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You attach a clear pipe to the bleed nipple and a receptacle for any fluid that comes out, and as you loosen the nipple on each caliper you'll see air bubbles/fluid coming through you stop seeing bubbles coming through the pipe. Once this fluid runs clear & free of bubbles you simply tighten the nipple back up and move to the next caliper.
 
This forum gives you a real education. I didnt know about air getting into brake fluid and making it go spongey before I joined here.

Everyday's a school day
 
Adamski said:
This forum gives you a real education. I didnt know about air getting into brake fluid and making it go spongey before I joined here.
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There are actually 2 potential problems with brake fluid.

One is getting air bubbles, as already mentioned, and is resolved by bleeding the brakes.

The other is that brake fluid is hydrophilic - it attracts water from the atmosphere. The issue with this is that while brake fluid is not compressible (which is why it is used in hydraulic systems), water is compressible. Hence having even a small amount of water in the brake fluid will result is a "soft" pedal. This is why brake fluid needs to be changed periodically.
 
and also the reason that any left over should be disposed off, once opened it has to be used straight away and it does as mention above even if the container is sealed..
 
Sorry to be a PITA, but... :D
Water is not compressible. As far as i know, liquids in general are not compressible. That's why you use hydraulic systems.
The problem with water is it's boiling point. Waters boils at 100°C (under normal pressure...). In a brake system, this temperature is reached quite easily. If you put water in your brake lines, the water will boil and turn into vapor, which IS compressible.
A brake fluid like RBF600 has a boiling point of 600°F / 315°C. So, if your brake system goes over 315°C, you will have the same problem of "spongy" pedal. Assuming nothing catches on fire first...
As you said, brake fluid is hydrophilic because you don't want any water in your brake lines for the reason stated above. But when the brake fluid absorbs water, it's boiling temp lowers (i don't know exactly in which proportion though), sometimes reaching a point where you get the spongy feel again.
 
You are, of course, correct - it's the vapour which is compressible, not the liquid, and brake systems can easily go well above the boiling point of water under even reasonably heavy/prolonged braking.

DOT5 brake fluid is not hydrophilic, but this causes other problems (potential corrosion of the braking system by any water that does get in).
 
ZedFourM said:
DOT5 brake fluid is not hydrophilic, but this causes other problems (potential corrosion of the braking system by any water that does get in).
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Exactly, i was refering only to glycol-based brake fluid, not silicon ones.
 
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