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[Rods2]

Never had a problem on various bikes and cars when pushing the pistons back, but I do tend to do this very slowly with a G-clamp and pieces of wood.

Thinking about it is does make sense to loosen a bleed nipple to get rid of the old fluid though and this will also stop the master cylinder reservoir overflowing, if it has been topped up with worn pads.

[TheBoy]

Never had a problem on any car...  ...but then I have always cracked the nipple open, as taught to me by my 2nd eldest bro when I used to get in the way help him with his cars...


Having seen DTM's pic, though, I have a clearer understanding of the problem. Great piccy

Dr.gixer, I think your idea of "flipping" is possibly wrong, going by your description

[Omegatoy]

FLIPPING IS I THINK JUST A GENERIC TERM USED BY THE TRADE,
although i have seen it in high mileage motors, generally if the seal is a little weak or old and worn it can be the last straw when having fluid pushed back on the most worn part of the bore of the master cyl,
i.e where the pedal moves most the first inch or so of the mc,? also it can force dirt and crap in the system past the weaker seals, so
I always undo the nipples and get the mostly boiled fluid out to be replaced with fresher stuff

[Magwheels]

Never had a problem on various bikes and cars when pushing the pistons back, but I do tend to do this very slowly with a G-clamp and pieces of wood.

Thinking about it is does make sense to loosen a bleed nipple to get rid of the old fluid though and this will also stop the master cylinder reservoir overflowing, if it has been topped up with worn pads.

It once took me about an hour to do one side on the rear of the wife's Golf mk4 as I didn't have the correct wind back tool so had to use pointed pliers and a lot of effort!! Very very slowly indeed ... That soon made me buy the right tool and from then all so easy. 

 Tis true on both counts as I'd rather loosen the bleed nipple than my wallet for a new ABS unit!!! 

[chrisgixer]

Never had a problem on any car...  ...but then I have always cracked the nipple open, as taught to me by my 2nd eldest bro when I used to get in the way help him with his cars...


Having seen DTM's pic, though, I have a clearer understanding of the problem. Great piccy

Dr.gixer, I think your idea of "flipping" is possibly wrong, going by your description

Idea? It's not an idea, its the dictionary

But as said by Magwheels, I'm struggling to see how pushing the calliper piston in can overcome that flange on the seal any more than pressing the pedal ?

Unless... If the seal is in a resting position that allows the flange to sit in a port or drilling that allows fluid in, then when the driver presses the pedal the seal moves past that port so it's blocked off under pedal pressure.

So then if fluid is forced back up by pushing the caliper piston in, then the flange on the seal would be able to invert.

But I can't understand it otherwise. Question has to be, how can pressure from the calliper piston cause more damage with less force, than pressing the pedal?

[Andy H]

FLIPPING IS I THINK JUST A GENERIC TERM USED BY THE TRADE,
although i have seen it in high mileage motors, generally if the seal is a little weak or old and worn it can be the last straw when having fluid pushed back on the most worn part of the bore of the master cyl,
i.e where the pedal moves most the first inch or so of the mc,? also it can force dirt and crap in the system past the weaker seals, so
I always undo the nipples and get the mostly boiled fluid out to be replaced with fresher stuff

This is the only explanation that makes sense to me.

If the bore is worn enough to allow a shagged piston seal to escape then I don't think want either in my car though 

[Temetsy]

I think the flipping happens when pushing pistons in because when you press brake pedal it moves away from the pressure and when you look at the flange you can see that it's pointing 'backwards' so when you push pistons back in not only the pressure of the slowly escaping fluid affects the seal's flange but the flange might grip to the cylinder walls and cause it to twist to the wrong direction and if pushed quickly it may flip.

[Magwheels]

I think the flipping happens when pushing pistons in because when you press brake pedal it moves away from the pressure and when you look at the flange you can see that it's pointing 'backwards' so when you push pistons back in not only the pressure of the slowly escaping fluid affects the seal's flange but the flange might grip to the cylinder walls and cause it to twist to the wrong direction and if pushed quickly it may flip.

? 

[Temetsy]

I think the flipping happens when pushing pistons in because when you press brake pedal it moves away from the pressure and when you look at the flange you can see that it's pointing 'backwards' so when you push pistons back in not only the pressure of the slowly escaping fluid affects the seal's flange but the flange might grip to the cylinder walls and cause it to twist to the wrong direction and if pushed quickly it may flip.

? 

The blue arrow indicates the force affecting the seal when brakes are used. The red is for the force/friction to the flange when pushed back in: Brake fluid resisting when pushed back and the flange of the seal against the brake cylinder wall (black lines). What I meant with the seal's flange gripping cylinder wall is propably best described when u think of a metal saw, it's teeth are pointing to one direction (like that flange is angled to point down the cylinder) and when the saw blade slides 'against the hair' it sure grips more than when sliding it to the other way. Hope this complicated explanation clarifies my even more complicated text  

[Magwheels]

Not really 

But the piston is in the resting position with the return port open, therefore the fluid would return to the reservoir. If the return port was not open you would have a very efficient handbrake foot operated park brake.