At 08:55 PM 1/22/97 GMT, you wrote:
>Hi Chris,
>
>The PDWA valve also closes the system that has failed. The valve is basically
>a shuttle valve. If one system (front or rear) were to fail the shuttle will
>shift to close the failed system and maintain function in the other system.
>The valve is shaped much like an H with the shuttle in the middle. If for
>example, you have a line fail on the rear system, the pressure differential
>from the front (normal pressure) to the rear (no pressure) will cause the
>shuttle to shift to the low pressure side. What this all means is that you
can
>have a line failure (or cylinder) and still have brakes on either the front or
>the rear.
>
>I wouldn't bypass this valve, half of a braking system is much better than no
>braking system.
>
>Good Luck
>
>Bill M.
>'70 MkIII Spitfire
>
Ummmmm. I'm not too sure. I agree, don't bypass the valve, but just on
general principles of not messing around with brakes (a ton of metal at
60 mph with no brakes contains a very great amount of power just looking
for something to damage). The reason I'm not so sure is because the cars
have a tandem master cylinder, at least my '71 TR-6 which had the PDWA switch
did; thus each system has its own mastere cylinder providing pressure.
In reading Bill's post, I got all excited at first, because I was thinking
that you could plumb a PDWA into an older, single master-cylinder system
in order to have a dual circuit system, as a safety measure. I was thinking
you could plumb the PDWA into the system downstream of the junction block
where the system splits the single line into the front and rear circuits,
with one circuit on either side of the PDWA, as it is on the cars on which
it came standard. Then if one circuit failed, the PDWA would cut out the
non-working circuit, and voila, you have a back-up as on cars built since
1967, when the Feds mandated it.
But when I got to thinking about it, I realized you still have to have the
separate master cylinders. If you only have a single master cylinder, even
with the PDWA in the line, if one circuit loses pressure, all the pressure goes
to the side with the least resistance, that is the side that blew out the
wheel cylinder, hose, or whatever. I think you've still got to have the
dual master cylinder.
Somebody please prove me wrong on this, I still would like to think that
it would be nice to have the safety of a dual-circuit cylinder.
Suggestions?
Mark Moburg
markmoburg@mindspring.com
57 Jaguar MK VIII
58 MGA
70 M*rc*d*s-B*nz 300 SEL 6.3 litre
85 XJ6 Vanden Plas
60 TR3 - one owner, black, overdrive, hardtop, all options. I know
where it is & I'm not telling :^)
|