Incorrect Anti Rattle Spring Installation and spongy brakes
 

Hi,
I just finished installing new pads on my S80. However I have wrongly installed (as i've now learned) the anti rattle springs.

I've forced them behind the lugs, (wrong)

I've spongy brakes now,

My question is? Why does this cause spongy brakes. Let's assume there is no air in the lines (brake fluid) and brake system. So if the brake pedal is spongy, it must mean there is some 'give' somewhere.

Can someone explain what is going on? I'm an engineer and it's bugging me that I don't understand why.

BTW, its dark now. I plan to correct my mistake tomorrow.

Thanks

I believe that, fitted correctly, the spring holds the caliper towards the disc and keeps the pads in contact with the disc. Installed incorrectly, it pulls the caliper away from the disc. So when you press the pedal, the pads have further to travel. I would think, the pedal would feel the same as the first time it's pressed after a pad change.
Gary

Here're are the before and after pictures.

Wrong Rattle clip install and the correct rattle clip install.

The pedal was spongy with the clip installed incorrectly.

The pedal was solid with the clip installed correctly.

Thanks

Great pictures which do suggest a possible explanation. With the spring fitted wrong the pad extends slightly farther out of the caliper (toward the axle) by maybe 5 mm than with it fitted correctly. So maybe with the wrong fitting the pad is contacted too far off center. When the caliper is pressurized maybe the pad rocks and doesn't contact the disc flat on.

One might be able to see this by observing the operation of the caliper with the spring in the wrong position. Of course, not many would think it worth it to jack the car up, take the wheel off, put the spring on wrong and have someone get inside and press the brake while someone observes the operation of the caliper. For one thing a static test might not reveal anything.

If it's any use, I did exactly the same when I fitted new pads and discs. I was worried about the spongy brakes particulerly as I hadn't touched the hydraulics and also noted the the pads were were only making contact with the outside 50% of the disc (as this was where the polish mars could be seen. Worked out my mistake and adjusted the springs - everything brill. I suspect that the springs do indeed hold the pads against the surface of the disc but not enough to make the brakes bind.

good pics just finished doing mine and very relieved to see ive done it right although it was a complete PITA getting it back in

ok, seeing the pictures I understand
the way you did it first the whole unit was pressed away from the disk and by doing that the piston was further more pressed inside
and therefore each time you used the brake pedal the first movement the piston had to make was bringing the pads to the disks again

spongy brakes?? and pedal travel
 

Back to the OP's original questions...

Has anyone been able to determine why you get the spongy feeling whenever the pads are changed? and then it goes away after 'seating' the pads/rotors... the physical dimensions between caliper, pad and rotor do not significantly change

And why is there added pedal travel occurring after a pad change? which also goes away after 'seating'

All of which, to me, is very confusing. anyone have any suggestions?

ps see other postings by me on caliper problems etc.

Cheers
Eric

sorry, forgot to tick auto notification button

Spongy Brakes
 

Just want to say a bigh thank you, changed discs and pads yesterday and new brake fluid, then spongy brakes, up at 7am this morning and rebled the whole system, still spongy brakes, could not figure it out and getting depressed, searching the web and hey presto found your article, whipped off the two wheels and sat the springs correctly, test drive and everything fine, thanks you've just made my day, smile back on face!! Regards Graham