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RE: Stainless Brake Pistons

To: triumphs@autox.team.net
Subject: RE: Stainless Brake Pistons
From: "Peter C." <nosimport@mailbag.com>
Date: Tue, 12 Feb 2002 08:56:00 -0600
 > Scott Tilton wrote:
 > > I would assume that any amount of tiny pitting or rust is cause
 > for a rejection of an used brake piston.  Is this correct?
        YES!!!!
Joe Curry wrote :
 > But the piston doesn't really contact the
 > cylinder but serves as
 > an housing for attaching the cups that do contact the cylinder
 > walls.  So as long as the piston does not bind up and still seals
 > the cup and/or O-rings, it will still work regardless of pitting.
------
        True only of Dunlop style calipers as on early Jaguars, early Datsun 
roadsters, etc.
----------
We need to be clear here about whether we're talking about front disc brake
caliper pistons, or drum brake pistons.
<snip>
For disc brakes, the seal is mounted in the caliper body, and rides on the
sides of the piston.  Any pitting or corrosion in this area will cause leaks
and rapid seal wear.
Randall
=============
If I may add to Randall's note: The seal is a square-cut O-ring and is 
designed to deflect when the caliper is pressurized and the piston moves. 
When the pressure is released (foot off pedal) only the seal relaxing 
releases the pad from rotor contact. (there's no return springs ala drum 
brakes) Any pitting of the piston surface will scar the seal and rather 
than flex, it will just scrape the piston and not assist in pad retraction. 
This is the beginning of accelerated pad wear. Insist on new pistons every 
rebuild (many rebuilders don't always change them) Personal preference is 
stainless, fwiw.  It's only brakes.
        Peter C.

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