John, (and other interested parties)
The PDWA is just as its says, a pressure differential warning actuator, not
a hydraulic valve but a hydraulic switch. The two brake circuits are
totally independent of each other. inside the PDWA is simply a shuttle
that sees both sides of the braking system pressure so that when everything
is normal the pressure on either side of this shuttle is equal. When there
is a pressure loss (or differential) on either side of the shuttle (from a
leak) then the shuttle simply moves within it's bore in the direction of
the lower pressure, causing the switch to complete the circuit that
supplies 12 volts to the warning light. It DOES NOT shut off the offending
braking circuit, the master cylinder is supposed to take care of that.
That's one reason you effectively have two master cylinders in one housing.
If it actually disconnected or blocked the function of the offending
braking circuit there wouldn't be any need for a dual master cylinder the
way it's configured because the "valve" would close off the leaking circuit
allowing the (single) master cylinder to still apply pressure to the
functioning lines.
The re-centering procedure only re-centers the shuttle, turning the lamp
off. If not centered on a perfectly good system, you would still have
normal braking but you would also have a warning lamp glowing all the time.
If not re-centered using the procedure after a failure of one side, the
shuttle sees the same pressure on either side in a perfectly normal system
and has no reason to return to it's "centered" position since there is no
difference in pressure on either side of it causing it to slide one way or
the other it just stays where it is. The switch's contact is what
sometimes prevents it from being easily centered.
There is of course an advantage to having redundant masters in that in the
event of master cylinder failure of one system your suppose to have a
working master in the other - but I won't go there :-) (I already have)
*********************************************
>Well, if the PDWA doesn't shut off the offending
>circuit, it certainly should!
>
>In fact, the procedure for re-centralizing the plunger
>suggests to me that the PDWA does shut off the faulty
>circuit... I thought my Bentley manual indicated that
>if you don't follow the procedure properly, you'll be
>missing out on the benefit of all your wheels braking!
Barry Schwartz (San Diego) bschwart@pacbell.net
72 PI, V6 Spitfire (daily driver)
70 GT6+ (when I don't drive the Spit)
70 Spitfire (long term project)
|