Boiling brake fluid Redux

Well, I'm no brake expert but I do understand exactly how they work and I'd venture an opinion that you can't tell anything that way...they're shoved back just out of solid contact by the rotating disc and the forces are so high that you can't judge how much is being applied by a freely spinning wheel...yours sound fine to me.

Sounds like you did it all right...

Well, I bought some tubing so i can make one of my flushing devices, and I'm going to at least flush that caliper later.

LOL...did I ever mention I used to work for an Aircraft/Aerospace Company? We made spacesuits, too. No rocket science involved! ;)

Sounds normal to me. The calipers aren't supposed to back off at all - there are no return springs in disc brakes like there are on drum wheel cylinders, but no residual pressure valve either.

The natural thousandth or two of runout in the rotor (along with a bit of natural slop in the wheel bearings) pushes the pads and calipers back from the rotor after you release the brake pedal and start moving. Which is why you hear a little squeaking and groaning right as you start moving.

With the car up on a jack and the tire being spun by hand, there is quite a bit of resistance till you move the pads back.

It's also why your system brake fluid volume goes up (and the level goes down) as the brakes wear - the caliper pistons keep moving further out.

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Bruce L. Bergman wrote in news: snipped-for-privacy@4ax.com:

A normally-operating brake caliper piston will retract a bit (1/32" or so) when you let off the pedal. This provides the pressure release function.

As gum builds up on the piston, the retraction action is reduced, then finally stops, resulting in a dragging brake.

We're likely arguing nits here but I don't see why the piston would 'retract' at all...it will be 'shoved back' into it's 'hole' slightly by any slight wobble or unevenness in the turning disc but as long as the caliper is free to move sideways and the piston is free to move in and out then the disc will shove the piston out of it's way (so to speak). I don't see any action in the system that would cause the piston to 'retract' (I equate 'retract' as the piston being 'pulled' back inside it's cylinder and not being 'pushed' into it)

Gord Beaman wrote in news: snipped-for-privacy@4ax.com:

It does. Go pull your caliper and look for yourself.

Can you explain why?

Gord Beaman wrote in news: snipped-for-privacy@4ax.com:

Don't know for sure. My guess is the return springs inside the master cylinder are responsible. In pushing the seals back, they suck the fluid back as well, and if the piston is free to move, it gets sucked in too.

But they *do* pull back in a bit after you let of of the pedal. I see that all the time. It's one way I have of telling what sort of shape the piston is in. If there's no pull-back, there's buildup of some kind on the piston.

Ok, makes sense...thanks Teg.

Before I read Teg's response, I'm going to take a guess: there will be some retraction (not much) when you take your foot off the brake pedal. Think of fluid dynamics: you've pressed the pedal, and introduced pressure into the system. Take your foot off the brake, and the release of pressure will cause a small amount of back pressure, sucking the caliper back in just slightly.

Just an educated guess...

Did you take out the two pins, clean them and then lub them with a good synthetic break lube?

Richard.

Pretty close Hach...as teg says, the master cyl piston is shoved back by a spring...now the seal on that piston has to 'unseal' so as to be 'ready' to 'grab' more fluid if it's necessary so there has to be a very slight negative pressure on the fluid in the system as it unseal and this is what moves the slave pistons slightly. Remember that there's four slave cylinders and they're all much bigger than the master cyl therefore they'll all move only a slight amount...that sound about right Teg?

Gord Beaman wrote in news: snipped-for-privacy@4ax.com:

There would only be negative pressure so long as the return ports are still closed One the seals pass the return port, pressure is equalized. And if the fluid warms up appreciably while it's compressed (such as when the brakes get hot), there will be POSITIVE pressure until the return ports open. That's how a misadjusted M/C pushrod causes the brakes to lock on as you drive.

I made up this page:

Could be. I'm not up on the physics of it. All I know is what I see.

See my subsequent posts...I...I've tried everything. Just giving up and ordering a new caliper and hose...