[Shotimes] More comments on SHO rear brakes (was proportioning valve question)

[Tom Leeth]

Thanks for the responses to my question on leaving the valve push rod lashed
in the fully upward position.  My SHO is a '93 ATX with 92K miles.  I live in
D/FW area of Texas and bought the SHO new.  I have chased a dragging right
rear brake problem for at least the last four years.  The dragging usually
occurred in slow turns, made just after the brakes had been applied and was
worse if the suspension had just been flexed a little (live pulling into a
driveway with a short uphill grade).  In that time the caliper, slider pins,
brake pads, rotor, brake line, and proportioning valve were replaced in
separate attempts to fix the problem.  Although the car is rust free, I double
checked the parking brake cables, adjusted the free play in the cable
assembly, and disconnected the cables from the caliper lever arms to make sure
that the dragging was not due to the parking brake not completely releasing.
Nothing fixed the problem.

I noticed over this time that the car often had some resistance to first
motion when I back it out of  my garage, even though the floor and adjacent
driveway are level.

With the proportioning valve lashed in the full open position, there has (so
far) been no resistance to first motion and no dragging right rear brake pad.
The proportioning valve was replaced about 10K miles back and the push rod
length was adjusted to provide more pressure than the factory setting, to the
rear calipers (as per Helms).  It is possible that the right rear hub/bearing
assembly that I just replaced had enough play in it to cause some of the
dragging problem in turns, but it seems unlikely that it could have caused the
resistance to first motion which for all the world felt like the parking brake
was slightly set.

Bottom line - the proportioning valve is a flow restrictor not a pressure
regulator, and it appears that the design is such that the flow restriction is
not the same in both directions.  The pressure rise on the rear pistons
happens very quickly when the brakes are applied, but the pressure drop is
much slower and in my case sometime takes many hours or even days on the right
rear.  If any of you are still chasing a dragging rear brake problem, try
lashing the proportioning valve in the fully open position using a couple of
nylon ties - it costs about a nickel and takes about 2 minutes.  If you don't
like the results, it takes even less time to put it back the way it was.
Please note, the SHO Guru's advise not to do this if you don't have a
functioning ABS on your SHO.

Tom Leeth