Plus 2 rear brake disk wear

[alan.barker]

ok fair point why take the risk
Alan

[JonB]

Well, I have some brake cleaning spray (highly flammable!) and will try to exercise the calipers a bit.

Only downside to undoing the bleed nipple is loss of fluid, but I was going to change it anyway.

I'm thinking about this master cylinder seal damage. Maybe the edge of the seal turning over a lip in the cylinder bore? I have done this before on Spitfires and not encountered the problem, but I do agree it's better safe than sorry in this instance.

To compress the pistons after extending I use a long plumber's wrench like this one: https://pixabay.com/en/plumber-wrench-p ... ol-885115/

It's one of my "magic tools". So many uses.

[MarkDa]

My take on bleeding off at caliper comes from advice on more modern ABS systems which are very sensitive to dirt.
So if there is any debris in the caliper rather than flush it back through the master cylinder to the reservoir it gets dumped out of the bleed nipple.

[JonB]

Another good reason to faff about with a bleed tube..

[denicholls2]

It's said that reversing the pressure causes the seal to 'flip' or otherwise be damaged but I can't really understand how that might happen so you raise a fair point.

Any system where that can happen is a critically unsafe system. The net pressures applied to the parts are the same as in-service braking regardless of which side is pushing. And surely you don't expect that result from in-service braking?

The diameters of the cylinders determine relative travel for a given push, but that's the only practical difference in which side is applying pressure. And as a small push on the pedal drives a larger movement at the wheels, so movement initiated from the wheels will drive a smaller motion at the cylinder. Making up for all of the wear in the pads in one push will mean more total motion but the likeliest place for that to go is the gaps at the other three wheels (wheel on a dual system).

Dual systems where an imbalance of pressure unseats a warning sensor are the best explanation of brake failure after push-back. And would be addressed by the procedure of replacing the master cylinder seal.

[joe7]

In my early learning stages fitting new pads I didn't take of the MC cap and blew a caliper seal or 2 pushing the pistons back. Why? Pushing back on the pistons means the fluid has to go somewhere. However with the MC cap on, and yes there is a small breather hole in the cap but not large enough to accommodate the amount of fluid being pushed back into the MC so the fluid has to go some where.

[alan.barker]

Use a good fitting spanner or 6 sided socket to open the bleed valve and be careful we don't want it shearing off
Alan

[JonB]

With a good soak in WD40 beforehand..

[mbell]

I think it is probably safest to open the bleed valve to do this, I don't think there any risk to good condition master seals from just pushing it back. With the brake pedal released the master cylinder is open to the reservoir so any fluid returning shouldn't create enough pressure in the master cylinder to damage the seals. The fluid should just flow back in to the reservoir.

Of course if there is a restriction in the system somewhere between the master and reservoir or the reservoir vent is very restrictive, the the pushing back could create pressure in the master that could cause an issue. Especially if the mater seals aren't in good shape.

All that said using the bleed nipple is the safest route and if your fitting new discs/pads why not take the time to replace the brake fluid anyway. It very low cost, not very difficult and critical to the safe operation of the brakes.

[Hawksfield]

Hi

If you cannot push a wheel cylinder back without causing a problem then there is something wrong in the system, when the foot pedal is at rest the seal to the float bowl is open allowing return of fluid to the bowl otherwise the brakes would lock on !. To have a seal flip in the bore is not possible unless the seal has tuned to mush.
I have never opened the bleeds when changing pads as it would require a re bleed of the system, not a simple task getting all the air out of the system on a +2

However this is the procedure that works for me

[alan.barker]

Imho,
if pushing the pistons back causes a leak with a seal it is because the piston is rusted or with contamination.
If the bleed valve is opened before the piston is pushed back then closed when you stop pushing the piston this will not mean you need to bleed the system. Air plus liquid has only been pushed out and no air let in if bleed valve closed straight away.
An advantage of opening the bleed valve is liquid that is expelled could well contain water that has collected in the Caliper.
Alan

[MarkDa]

Sometimes you do need to bleed off anyway when changing pads.
If the reservoir has been topped up when pads are well worn retracting the pistons to both calipers can easily csuse an overflow.
In my book this is standard procedure for all of the reasons already given.
As Alan says carefully done, especially with a one way bleed tube, there should be no need for a formal pedal driven re-bleed.
Although a push or two to seat the pads and then let a little fluid through to check that there's no air in the caliper while the tube is attached makes sense.