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Message started by Andy RN on 04/05/18 at 14:20:31

Title: What pushes brake piston back?
Post by Andy RN on 04/05/18 at 14:20:31

Hi all! Can anyone explain what moves the piston in the front brake caliper assembly away from the rotor after the brake lever has been released? Looking at the fiche, I don't see a spring in the caliper that would push the piston back, and it seems surprising to me that it would only be the pressure of the brake fluid flowing back towards the master cylinder - or is that really it?

Also, I saw a video of a guy replacing brake pads, and he used his hands to push the piston back into the calipers (to make room for the new pads). I just replaced my pads for the first time, by myself, and I had to use an old pad and a c-clamp (as was recommended in the tech section). Everything worked like a charm, and I replaced the pads and the brake fluid and things are working well, but I'm just wondering if I should have been able to move the piston with my hand? The bike only has 5K miles on it, and it wasn't super gunky when I replaced the pads. Should I add a caliper breakdown, clean, lube and reassemble to my list of things to do?

Thanks!

Title: Re: What pushes brake piston back?
Post by oldNslow on 04/05/18 at 17:23:36

this is a pretty good explanation:

https://mechanics.stackexchange.com/questions/21579/how-do-disk-brake-pistons-retract

The animation shows a double sided caliper - one piston on each side of the disc. The caliper is fixed in place. The pistons, one on each side - each retract and let the brake pads stop pressing on the disc. On a single sided caliper, like the one on the savage, the caliper can move. When the pad that is pushed by the piston is pressed against one side of the disc, that movement pulls the caliper toward the other side of the disc and pulls the pad on that side into contact. When the brake lever is released, the piston side pad retracts, and the pad on the opposite side is no longer pulled against that side of the disc.

The movement is very small. The pads are always lightly contacting the disc. They just aren't squeezing it until the lever is pulled and hydraulic pressure is applied to the caliper.

The "gasket" that the animation refers to is more commonly called a piston seal. It looks like a big square O-ring.

Title: Re: What pushes brake piston back?
Post by Andy RN on 04/06/18 at 14:04:25

Thanks, oldNslow.

The stuff you say makes sense, and it echos what Verslagen wrote in a post in the tech section about replacing the pads:

"In theory, the caliper is free to slide in and out on the posts to adjust for variations in the rotor. Also, since only the outside brake pad gets pushed, there needs to be some slack in the caliper mount to allow it to squeeze and release. "
http://suzukisavage.com/cgi-bin/YaBB.pl?num=1177184106/5#5

So it sounds like a significant amount of the force to disengage the pads from the rotor IS based on the negative fluid pressure. It's kind of wild to think that a single pump on the brake lever, and a single cycle of the piston in the master cylinder is enough to create that much force. The miracle of fluid dynamics! Do you think the Piston seal in the calipers also has a significant role to play in pulling the caliper piston back?

I wish Haynes made a book like this about the different kinds of motorcycle brakes:

https://haynes.com/en-us/motorcycle-fuel-systems-haynes-techbook-haynes-repair-manual

Thanks again, Andy.
PS - my handle on this website could well have been "middle-agedNslow" :)

Title: Re: What pushes brake piston back?
Post by Ruttly on 04/06/18 at 19:39:39

Andy you are correct , the piston seal is a square o-ring when pressure is applied it changes from square to a paralellagram( spelling sucks ) and when pressure is released it is the rubber seal that pulls piston back. The caliper floats on the bolts allowing even wear to both pads.

Title: Re: What pushes brake piston back?
Post by Armen on 04/07/18 at 06:47:25

Trivia:
I was working on someone's 70's CB Honda a while ago. As usual, the front caliper was garbage. Rust, corrosion, pitting, etc.
Told him to buy a repro caliper on Ebay. Pretty cheap, and came with new bleeder and pads.
Put it on, and not for love nor money could I get decent lever pressure. Considering that the system has only one more moving part than a cinder block, it had me wondering. I had just read an article by Kevin Cameron where he interviewed the guys at Brembo, and they said it was all about the piston rubber seals/o-rings. Bought a factory Honda o-ring and installed (perfect fit, as the part was an accurate repro). Voila! Lever presser almost immediately.
I guess the folks at the Sum Flung Dung machine works in China were better at the metal part of the repro than the rubber seal part.
The next one I bought (different supplier) worked like a charm right away.