[DeTomaso] Stock Brakes

[SOBill at aol.com]

This may be known by all, but:
 
The pistons in disc brake calipers automatically adjust the pad  position 
relative to the rotor in a quite subtle way.  As the fluid moves  the piston 
toward the rotor the piston seal is slightly deformed to drag along  behind the 
actual piston displacement. When the fluid pressure is released, the  slight 
deformation of the seal will move the piston away from the rotor a very  small 
amount. The rotors have very little run out, so the pads remain very near  the 
rotor. As the pads wear, every application of the brakes repeats  this cycle 
and the pads creep toward the rotor to keep the brakes  adjusted.
 
You can monitor the pad wear by tracking the fluid level in the  master 
cylinder reservoir. As the pads wear, the brake fluid level will drop.  When you 
replace pads, you force the pistons back into the caliper and the fluid  level 
in the reservoir returns to normal. If you periodically top up the brake  fluid 
in the reservoir, the next time you replace pads you can easily overflow  the 
reservoir when forcing the pistons back into the caliper. Brake fluid does  
not evaporate, so, if you have no leaks, you should never need to add brake  
fluid.
 
If the piston bores are rough, or the pistons were assembled  without brake 
assembly lube, making it difficult for the seals to slide, the  pads will not 
get to the correct position relative to the rotor and you will  have a longer 
brake pedal travel. Keeping the bores clean and free of dirt is  the primary 
purpose of the dust shields on passenger car brake  pistons.
 
If the rotors have excessive lateral run out, on each rotation the  piston 
will be pushed back into the caliper as the high spot goes by and will  stay 
there. This means that the pistons must be moved by considerable pedal  travel 
the next time the brakes are applied.
 
SOBill  Taylor
sobill at aol.com



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