Brake bleeding baloney - or not (OT) |
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Fooljam
Senior Member II Joined: 16-July-2008 Status: Offline Points: 207 |
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Posted: 31-January-2010 at 20:59 |
There you go, that's your problem You do not need that crap The seal only is here to prevent brake fluid from leaking between the cylinder and the servo. I do not see how come by using a pressure equivalent to pressing a door bell with your finger you would destroy this seal. Your cylinder most likely is dead, if your seal is dead too, you should have fluid leaking around here. Anyway, it is a good idea to replace both. My 2 cents. |
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b318isp
Moderator Group Joined: 10-October-2002 Location: Ireland Status: Offline Points: 2057 |
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But reverse loading is the same direction of pressure. If the pressure was coming from behind the seals, so to speak, I'd understand. |
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Andrew Rolland
Moderator Group Go away rain Joined: 19-August-2004 Location: Scotland Status: Offline Points: 6579 |
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Aye but to some extent the seals are shaped like one way valves so they will work under pressure one way (stomping on the stop pedal) but not so much under reverse loading like when pistons are being pushed back. I took fright on learning what damage debris in the brake lines can do to ABS units when I read up on pad changing on my E39 at the time so I decided to do it by the book so to speak. It wasn't a big deal at the time but I would have fluid on site to top up. |
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Current '13 62 F10 535i MSport Auto Previous '04 04 E60 545i SE Auto '03 53 E60 545i SE Auto (Stolen) '98 S E39 523i SE Auto |
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b318isp
Moderator Group Joined: 10-October-2002 Location: Ireland Status: Offline Points: 2057 |
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Interesting to hear that this isn't unusual. I still don't get how damage can be done though. Any pressure caused by pushing back the calipers would be less than the pressure built up under normal braking - hydraulic pressure is hydraulic pressure! A caliper piston wouldn't displace enough fluid for grit to get back to the cylinder (and it would have to come from outside too)...
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Andrew Rolland
Moderator Group Go away rain Joined: 19-August-2004 Location: Scotland Status: Offline Points: 6579 |
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Yeah Only ever retract pistons in calipers with the bleed screw open. Tells you to do this in most Haynes Manuals IIRC. It certainly did for the brakes on the E39. When I did my pad and disc replacement on my E39, I opened the bleed screw and was able to push the piston back by hand/finger pressure with ease, did get a bit of a jet of brake fluid out the nipple though. I then replaced the brake fluid and bleed it thru so I was satisfied that the system was minus air. You don't want grit/dust etc from the caliper etc getting its way into the fluid lines and possibly to seals which will damage them. Andrew |
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Be exclusive and drive a Petrol 5 Series!
Current '13 62 F10 535i MSport Auto Previous '04 04 E60 545i SE Auto '03 53 E60 545i SE Auto (Stolen) '98 S E39 523i SE Auto |
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denishogan
Really Senior Member II What’s this title thing about then? Joined: 29-March-2005 Location: Limerick, Ireland Status: Offline Points: 814 |
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A friend warned us about this a few years ago, just before we went to change the pads in a 2000 Astra. Knowing the guy and his experience, I would believe him. Can't remember the procedure though.
They just don't make cars like they used to! |
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b318isp
Moderator Group Joined: 10-October-2002 Location: Ireland Status: Offline Points: 2057 |
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My wife's Alhambra's brake pedal isn't great, too soft and it will slowly sink to the floor when held under steady pressure. Give it one pump and it's fine though. Classic symptoms of master cylinder seal problems.
Checking on the different forums, there is some evidance that the master cylinder seals can be damaged when replacing pads. It seems that when pushing back the calipers, the relevant bleed nipple should be opened - rather than pushing fluid back to the master cylinder's resevoir. Now, I've been bleeding brakes for a long time and I've never come across this. Apparently, pushing the fluid back to the resevoir may "invert" the cylinder seals. Sounds like a load of tosh to me, but I've seen it mentioned in a number of places. I also have changed pads, front and rear, over the last two years. So, let's say, it's possibly true. Has anyone heard of this before? I can't think of any other cause of the symptoms, other than the seals giving trouble. I'd like to be reasonably certain before I spend the money (and go to the hassle) of replacing it... ...and before you say it, the system is well bled with a pressure kit... |
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