brake piston drag/resistance

Joined
May 28, 2011
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Newhaven UK
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st 02 & 03
Hi folks,
Had a search in the forum but cant find anything about calliper piston feel/drag/resistance.......when servicing/replacing/cleaning the piston all and any, how much resistance should there be. With new seals a clean bore and a light amount of fluid to get them stetted, how hard/easy should it be to push them home or indeed pull them out to simulate the action of break fluid. I understand that the break fluid is under pressure but is there a guide line rule of thumb that can be applied with out putting it all together only to find it don't work.

thanks
 
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Re: break piston drag/resistance

I don't think there's any specification on anything, you're basically pushing a metal cylinder into a hole, there's not much to it. I think the only question is does it leak or not when you're done.
 
Joined
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Re: break piston drag/resistance

If you have the caliper completely detail stripped, take a small piece of wood or soft plastic and clean out the very back of the calipers where the pistons sit. Crap gets pushed back there when people push the pistons back in when changing their pads.
When replacing the seals, make sure you use a small amount of silicone grease on the seals, and as you stated, Dot 4 brake fluid on the pistons themselves to help insert them.
As far as "how they feel" going in, they should simply push in firmly, yet smoothly, and you should be able to move them easily without any pressure on the system. However, once you fill the system and start to bleed it, you should have firm pressure on all of them, and the pressure will only lesson when you open a bleeder valve to bleed the air out.
 
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thebigone
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132
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Newhaven UK
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st 02 & 03
thanks I get that, do you think you should be able to withdraw a piston with just finger pressure. I have seen a few clips on YouTube where the individual is using a G clamp to push the piston in....now that was for a car :eek: and again I know that the caliper on a car would be larger, but that kind of pressure is too much?

I always get a bit tetchy when I come to putting the pistons back in and getting them past the first ring they seem to be very tight and worry that I will trap and cut a small bit of the ring off and leave it in the caliper ready to move around the system.
 
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Use some rubber gloves and gently twist or rotate them as you install them, that way you get a better feel of them going in smoothly, and less likely to cut or tear anything.
.02
 
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If you are pushing a piston into an empty (of brake fluid) caliper after a rebuild, the resistance comes from compressing the air behind the piston if the bleeder is closed and friction from the rubber ring in the caliper body around the piston. If you are talking about changing the brake pads, and you are pushing the pistons back to accommodate the new thick pads, yes, a C clamp is often warranted. In this case you are (hopefully not) scraping crud off the piston as you push it in, overcoming the friction of that rubber ring, and forcing the brake fluid back through the system and into the reservoir (keep an eye on the last item so that it does not over fill and pour out onto painted surfaces.

Last time I rebuilt a caliper I noticed the piston's 'corners' were rounded. Obviously the engineers spec'ed that to allow the piston to gently push the rubber ring aside as you insert the piston into the bore.
 
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When changing brake pads, assuming you've cleaned them first, you don't need to use a C clamp, all you need to do is simply attach a hose to the bleeder and crack it open just enough to allow a small amount of fluid to escape, this will allow you to push the piston(s) in gently by hand without using force. When your finished simply add a little more fluid.
 
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thebigone
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May 28, 2011
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st 02 & 03
thanks for the feed back......just to clarify....I have taken off all my calipers, cleaned them, the seals are new, and the pistons nice and clean. So when I reassemble I should be able to, with light presser push the pistons home with the assistance of grease and fluid so they are seated correctly, and conversely pull them part the way back out again with reasonable pressure using thumb and fingers.

I do, and get the information about changing pads and allowing the piston to come forward a tad to clean the piston prior to pushing them back and having the reservoir cap(s) off to prevent back pressure building up
 
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