In a message dated 10/30/2005 8:51:26 AM Central Standard Time,
guy@genfiniti.com writes:
> Just got my right front brake caliper rebuilt (new seals, pistons,
> etc.) After getting it all back together I performed a test and put
> about 100psi of air through the system.
> The piston on the same side as the air feed came out strong right away.
> The opposite piston did not. I tried a second test where I forced the
> moving piston to stay in by clamping it. On this second test, the
> opposite piston did come out, but won't go back in as far as the other.
>
> My conclusion is that the pistons are not equal in terms of how easily
> they travel in the cylinders. Either one is too "loose", or the other
> too "tight". This leads to some questions:
>
> 1. Is there a sure-fired way of determining if a piston is "tight" or
> "loose"? How do you know?
> 2. Is this a big deal? In the real world on the car, I know that a
> balance will be achieved by braking power. There is a single fluid
> channel within the caliper that means both pistons will receive equal
> amount of pressure.
> 3. Or, is number 2 completely wrong, and the reason the one is coming
> out easier is because its nearer to the pressure source, indicating that
> equal pressure is not being applied on the opposite-side piston?
>
> One more piece of information. Although both pistons took some light
> "persuasion" to drive home in the cylinders, the one that is "tight"
> took more than the other.
> Any helpful tips and/or tests would be appreciated.
>
100 PSI in a system that normally operates at a thousand or more is not much.
The fact that they move at all is a good sign. The fact that they move with
different amounts of force may be more due to the amount of lubrication on
the rubber to piston interface. Fill them with brake fluid, drain and repeat
the test ans see if it changes.
2 is correct. Both sides receive the same pressure. But the pressure
required to move the pistons is trivial compared to the pressure applied to the
pads
when braking.
Dave
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