Brake bleeding baloney - or not (OT)

b318isp wrote: ...and before you say it, the system is well bled with a pressure kit...

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.

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.

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

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...