In a message dated 97-05-20, Daniel Haynes wrote:
> If it lights up during test, your brakes are
> supposed to be working. How this switch knows, I don't know.
Daniel:
Pressing the brake switch doesn't tell you that the brakes are working, only
that the brake failure warning light is working. The brake failure warning
light is wired to the PDWA -Pressure Differential Warning Actuater - switch
in the brake lines. One side of the switch assembly is plumbed to the front
brake line, and the other side is plumbed to the rear brake line. If
everything is ok, pressing the brakes will apply equal pressure to both the
front and rear lines. With equal pressure applied to both lines, the warning
switch remains centered in the assembly. If one line has failed, the pressure
in that line will be low, while the pressure in the other line remains high
while the brake pedal is depressed. This difference in pressure causes the
switch to move, turning on the warning light.
For a description of how this works, see -
www.vtr.org/maintain/pdwa.html
This describes the unit as used in a triumph. The unit itself is the same in
both the MG and the Triumph, only the electrical portion is different. In the
Triumph, the testing is done automatically when the key is turned on.
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition
'68 MGBGT---organ donor for the '74
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