That's fascinating, Jim. Thanks for that.
Anthony Giorgianni
For everyone's benefit, please post back to the group
"Jim Warman" <mechanic@[EMAIL PROTECTED]
> wrote in message
news:WM7Vj.1744$KB3.869@[EMAIL PROTECTED]
> Your "hold off" valve was actually called a metering valve. This was
> required on vehicles with a combination of front disc/rear drum brakes.
> This was to keep the front brakes from applying before the rears....
This
> was accompl;ished by "metering" the amount of fluid sent to the front
> brakes - limiting it, if you will, the movement of the front calipers
> until the rear drums "caught up". What it never did was to cause the
rear
> brakes to apply "first", rather, it's purpose was to have the brakes
apply
> in unison. Which brings us to the pro****tioning valve....
>
> The proprotioning valve was there to limit the pressure to the rear
> brakes. The harder we brake, the more weight is ****fted to the front
> wheels... If we allow the rear brakes to lock up early, the car will
swap
> ends...
>
> Notice we haven't mentioned the residual pressure valve.
>
> Some vehicles used a "combination valve". This was a metering valve plus
> pro****tioning valve combined into one unit. But the theory is still the
> same and it does not involve applying any one axles brakes "first".
> Something like that is going to be a nightmare from a stability
> standpoint.
>
> With the advent of 4WABs, all of these functions can be incor****ated
into
> the hydraulic control unit.
>
> Misunderstanding both vehicle systems as well as vehicle dynamics can
lead
> to all kinds of problems... Things "these days" work the same as they
did
> "in the old days" as far as dynamics are concerned... Depending on the
> system, technology may achieve these conditions in a mechanically
> different way...
>
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