>Well, if the PDWA doesn't shut off the offending
>circuit, it certainly should!
There seem to be a few kinds of PDWAs out there and they are normally
matched to a master brake cylinder.
>From what I have been able to observe so far there seems to be two basic
types of dual master brake cylinders made by Gurling/Lucas/TWR.
Both types rely upon fluid pressure for the mechanically activated piston
(near circuit) to puch the other piston (far circuit).
One type has the same dia cylinder front and rear and has a rod of some
kind between the pistons that will allow the pedal activated piston to
push the other piston if there is no brake pressure in the near circuit.
A PDWA on this kind of system only needs to tell you that there is a
circuit without pressure. WHile the piston crosses through the low
pressure side of the PDWA it doesn't NEED to completely block the flow of
fluid for the other circuit to be activated (read fewer '0' rings cheaper
to machine and build).
Another type of master cyclinder feeds a different number of wheel
cylinders between the front and rear circuits and has a larger diameter
piston on the far circuit to feed the dual wheel cylinders. The pistons
are not concentric and there is no rod for the mechanically activated
circuit to activate the far circuit. This one requires the PDWA to seal
the mechanically activated circuit in case of failure. Otherwise neither
circuit would work if the mechanically activated circuit would fail.
Land Rover dual circuit brakes work this way. The early dual brake short
wheelbase models (same number wheel cylinders at each wheel) do not even
have a PDWA. Long wheel base Land ROvers have dual wheel cylinders on
the front and a single on the rear. They require a PDWA.
I don't know about MGBs, but I do have some experience. I purchased a
'68 MGBGT that had a very recent brake job. After a little driving I
thought the brakes a little weak. I adjusted the rears & went to bleed
the system to discover no fluid in the rear cylinders. The PDWA was
closed and the rear cylinders were dry. Everything worked fine when I
reset the PDWA and prperly bled the system.
There seems to be a large variety of Gurling/Lucas/TWR PDWAs that came on
LBCs. I assume there is a reason and that not all PDWAs are the same and
I bet there are more than the two different kinds of actions I have
noticed.
All I know is while a PDWA may just activate a light in one model of car
it is essentual for brakinging during system failure on another.
TeriAnn Wakeman If you send me direct mail, please
Santa Cruz, California start the subject line with TW -
twakeman@cruzers.com I will be sure to read the message
http://www.shadow-catcher.net <- Photography for sale
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"In the world of type A & type B drivers consider me a type C gypsy
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