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Car Forum / BMW Cars / December 2004Soft/Spongy Brakes
Hello All, I replaced front pads and rotors today on the 1993 740i (116K). Job went very smoothly. The pads were down to about only 10% meat left. The brake pedal has been very spongy lately and the pedal travel is almost to the floor before you get any bite. I thought that once I replaced these well-worn pads, the pedal would come up, but it didn't. Here is my plan: Step 1: Check the rear rotors and pads, though I looked about 6 months ago and they had about 60% wear. If they still have some meat, I will proceed directly to step 2. Step 2: Bleed the entire brake system. I hope that one of these get it for me. My google searches have mentioned bad master cylinders as culprits for soft brakes--any thoughts? I figured I would post and see if anyone else has had any experiences like mine. Cross your fingers that bleeding these lines will do the trick. ER > Hello All, > [quoted text clipped - 22 lines] > > ER Sure sounds like you need a bleed, I'd go for that first, after replacing the fluid. If you track your car you're not allowed fluid older than 6 months by BMWCCA driving school rules. I'd go no longer than 2 years on the street. Once it gets contaminated all the bleeding and new pads in the world won't cure it. -Russ. > I replaced front pads and rotors today on the 1993 740i (116K). Job > went very smoothly. The pads were down to about only 10% meat left. > The brake pedal has been very spongy lately and the pedal travel is > almost to the floor before you get any bite. > I thought that once I replaced these well-worn pads, the pedal would > come up, but it didn't. Usual reason is air in the system. If you're unsure of the age, change the fluid. It should be done every two years anyway. However, air in the system usually indicates a fault elsewhere as it has to come from somewhere. And as you say the likely cause is the master cylinder.  Signature*Can fat people go skinny-dipping? Dave Plowman dave@davenoise.co.uk London SW To e-mail, change noise into sound. Not necessarily. It's recommended that brake fluid is changed every two years simply 'cos it degrades (by design). A faulty master cylinder won't necessarily introduce air - it just won't maintain pressure. > It's recommended that brake fluid is changed every > two years simply 'cos it degrades (by design). It absorbs moisture. Not through design, though. > A faulty master cylinder won't necessarily introduce air - it just > won't maintain pressure. Not necessarily. But if there are no signs of leaks, and bleeding doesn't cure things it would be my first guess. YMMV. Could be the last fluid change wasn't properly carried out and some air remains. Who knows? Signature*When blondes have more fun, do they know it? Dave Plowman dave@davenoise.co.uk London SW To e-mail, change noise into sound. Master cylinders can pull air from atmosphere on the release of the pedal. The fluid absorbs moisture from the air which impairs it's hydraulic capabilities, so could be either really. Best do the fluid first and if it is OK after that for a while and then goes soft again it will probably be the cylinder. Good luck! Clive Moisture is absorbed by the fluid *by design* to avoid the water vapourising when the brakes get hot and then and only then impairing the hydraulic properties. You could use water in your brake lines if they never got hot & you weren't worried about corrosion. Vapourisation of old fluid and/or corrosion can lead to gases in the fluid (which leads to corrosion etc etc) > Moisture is absorbed by the fluid *by design* to avoid the water > vapourising when the brakes get hot and then and only then impairing > the hydraulic properties. Then silicone brake fluid could never work, as it doesn't absorb moisture. Signature*Heart attacks... God's revenge for eating his animal friends Dave Plowman dave@davenoise.co.uk London SW To e-mail, change noise into sound. Some people don't use it for this very reason. And a spongy pedal! Moisture is absorbed by the fluid *by design* to avoid the water vapourising when the brakes get hot and then and only then impairing the hydraulic properties. You could use water in your brake lines if they never got hot & you weren't worried about corrosion. Vapourisation of old fluid and/or corrosion can lead to gases in the fluid (which leads to corrosion etc etc) Soft, spongy brake pedal is usually a sign of air in the system. > Hello All, > [quoted text clipped - 22 lines] > > ER |
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