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Brakes - |
6 December 2000 |
You can only use the MS72 adjustable brake
regulator/bias valve with a front/rear split system. Definitely NOT the
diagonal split type (for identification see 'Brakes - Rear brake bias
adjustment'). These should have been 'X-ed' at birth. Makes
the car handle very interestingly when the system only operates the diagonally
opposed brakes. Scary stuff. Almost as much fun
as when the rears only work on a front/rear split set-up - particularly as the
PDWA is built into the master cylinder, and it still has to go through the
FAM7821 pressure regulator valve! Damn near no braking on the rear. Either way
- split system brakes are grossly inefficient when only using half the system
as they are designed to do when a failure in the other half occurs. Quite frightening.
To install the MS72 into a front/rear split system, it
needs fitting into the rear feed line. Determining which this is can prove
difficult if the system has already been tampered with or new replacement
components fitted. Namely the master cylinder.
If the FAM7821 regulator is still fitted and connected
up to the master cylinder - then it's easy. The feed-line that runs to the rear
will be connected to the right-hand lower port (as viewed from under the
bonnet) of the regulator. Simply trace the feed pipe from the adjacent upper
port to the master - that will give you your feed port to the rear brakes
irrespective of whether the master cylinder is an early or late type.
Consequently the other one will be the front feed. Make a note of this some
place safe.
If tampered with - either bits replaced/removed that
aren't standard or are missing altogether - it's a little more difficult. True,
you have a 50/50 chance of getting it right, but 'guess-ology'
where the braking system is concerned is very irresponsible. Early-type
front/rear split system master cylinders used the conventional Imperial
threaded fittings. In these units, the lower/bottom port feeds the front
brakes, the upper/top port therefore feeding the rear brakes. The
later master cylinder (GMC227 - supplied as a replacement for the early types
and requires a conversion pipe kit, sometimes identifiable by a yellow plastic
band around the cylinder body) has metric threaded fittings with nuts of two
different sizes - top one bigger than the bottom one. The conversion pipe kit
is necessary because the piston functions (and therefore the port supply ones
too) are reversed. The upper/top port feeds the front brakes, the
lower/bottom one feeding the rear brakes.
Incidentally - something to watch out for - the two
thread sizes are very similar. I have seen a number of cars where someone has
managed to screw the imperial threaded unions into the metric ones. Apart from
this being a potential disaster, the wrong pistons in the master cylinder are
operating the wrong half of the braking circuit if the cross-threaded unions
stay fluid-tight that long!
Having identified which port does what, scrap the
FAM7821 if it hasn't already been junked. Connect the front brake feed pipe
directly to the front brakes, connect the rear brake feed to the front of the
MS72 (into the middle of what looks like a big nut), blank off one of the side
ports and take a pipe out the other side port to the rear brakes. I generally
mount the MS72 to the bulkhead stiffening triangle that braces against the
inner wing - convenient for connections and adjustment.
Useful part numbers:
|
MS72 |
Adjustable brake pressure regulator valve |