Jay, Dick, et al,
I believe that there has been some misunderstanding of how my "modified"
PDWA actually works. First of all, I would NEVER suggest that we eliminate
the separation between front and rear brake systems. The setscrews in my
PDWA very effectively seal each system from the other as well as sealing
each side from the "dry" middle of the PDWA where the switch screws in,
thereby maintaining the very important redundancy of the split braking
system. I can drain the brake fluid from either the front or rear systems
and still have braking on the other end of the car. Jay, I do not
believe that the PDWA functions as a proportioning valve. (Note: the UK and
other non-U.S. TR6s did not have PDWAs.) The PDWA simply slides to one side
or the other to trip the warning light switch when the pressure is
significantly different from one side to the other (i.e. when you have lost
fluid on one side). A proportioning valve, on the other hand, has to have
some sort of active unit that changes the pressure differential between the
back and front systems in relationship to increasing pressure by the
operator. Often, this is a pressure reducing valve to prevent the pressure
in the rear system from climbing at the same rate as the pressure in the
front system. This reduces the tendency to lock up the rear brakes in hard
stops. However, the TR6's PDWA doesn't change pressure on the rear system
as the pressure increases - its piston only moves when one side has lost
pressure completely. You can verify this by moving the PDWA piston from one
side to the other (you can do this by unscrewing the warning light switch
and using a bent piece of wire to move the piston) and then demonstrate that
the brakes feel exactly the same with the PDWA piston in any position.
It is my contention that the only purpose of the PDWA is to actuate the
warning light when pressure is too low to actuate the brakes on one end of
the car or the other. The only safety concession I have made in modifying
my own PDWA is to eliminate the warning light from illuminating when
pressure is lost on one system or the other. Remember, the light won't
illuminate until you push on the brakes and the pressure differential from
the back to front is great enough to move the PDWA piston and trip the
switch. By then, your foot will already have told you that you have a brake
problem!
Rex Townsend
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