[DeTomaso] Poor brakes

[MikeLDrew at aol.com]

In a message dated 7/10/09 12 45 46, asajay at asajay.com writes:


> 
> Now, if it was a totally enclosed system.  If you increased the 
> pressure on once side it should move the shuttle versus the lower 
> pressure on the other side.  However,  I've been told that liquids do 
> not compress, so no matter how high you change the pressure on one 
> side, in a totally enclosed system, I don't think you'd get any 
> movement of the shuttle.
> 

This has become rather academic, but if you had dual independent master 
cylinders (as opposed to the stock tandem style), and you applied pressure to 
only one of them, for sure it would displace the shuttle valve.   Doing so 
would result in a miniscule increase in pressure on the low-pressure side due 
to the very small size of the end of the shuttle valve acting as a 
piston--if indeed there was any pressure increase at all.

Brake fluid does compress a little bit, as do all fluids under pressure.   
If you were to measure the column of brake fluid in the hard line where it 
attaches to the master cylinder, when you stepped on the brakes hard, that 
fluid might move a couple of inches?   (That's a guess).   But down at the 
caliper, the movement of fluid in the line entering the caliper would be 
measurably less.   The difference is a function of the compressability of the 
liquid.

Forgetting for the moment the automobile which has multiple calipers with 
multiple pistons being acted on by a single master cylinder, consider 
something like a motorcycle brake.   Old-school motorcycle disc brakes have a 
single piston acted upon by a single master cylinder.   The piston's movement is 
all but imperceptable to the human eye, but the master cylinder moves quite 
a bit.   Although much of that difference can be attributed to the different 
sizes of the master piston versus caliper piston, the 'spongy' feel in the 
brake lever is due to the fluid (which often is contaminated with air, 
moisture etc.) compressing.

Mike


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