Rear brakes on lifted 80's
When the rear of an 80 is lifted, it changes the position of
the rear brake proportioning valve. This valve modulates the brake
fluid pressure to the rear brakes and compensates for varying rear
loads. As the rear load increases, the rear springs compress and
the rear axle moves
up in relation to the body. The proportioning valve senses this
shift and allows a higher pressure to the rear brakes. By lifting
the rear with taller springs, the proportioning valve is fooled
into thinking there is a light load and hence dropping the
pressure to the rear brakes.
Firstly, identify the rear brake proportioning valve shackle
mounted to the rear diff housing (as shown in the first figure.
The upper shackle has a rod attached to it which extends to the
proportioning valve. Under no circumstances should this rod
be bent for adjustment (however, there is a slight bend in
it from the factory).
The factory recommended adjustment is one which requires a
brake line pressure gauge to be fitted to the front and rear brake
bleed bolts. The steps are as follows:
Set the rear load at 1,150 kg (2,535 lb) and raise the front
brake pressure to 80 kg/cm2 (1,138 psi).
The rear brake pressure should be 45 kg/cm2 (640 psi) for drum
brakes and 61 kg/cm2 (869 psi) for disk brakes (including
disk braked ABS).
To adjust the rear brake pressure, one needs to adjust the
length of the lower shackle as shown in the second figure. The
factory length A is 90 mm.
If the rear line pressure is low (as we would expect for a
raised vehicle), the lower shackle needs to lengthened. This is
done by disconnecting the lower shackle from its bracket and
One turn changes fluid pressure by approximately 1 kg/cm2 (14.2
If you can't get enough pressure by lengthening "A",
you can adjust by loosening the proportioning valve itself and
sliding it up or down along its slotted holes.
For those with no access to pressure gauges, it is
unfortunately a matter of trial and error. It depends entirely on
the lift. Mine's the heaviest ARB rears (864's) with Kaymar rear
step, Long Ranger 170l sub tank and roller drawers. The length A
is 96 mm. The front wheels still lock first and rear pad life is
approximately 2 times the front, so this seems okay.