| When the rear of a NON ABS equipped 100 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 axle load at 1,500 kg (3,306 lb) and raise the front brake pressure to 80 kg/cm2 (1,138 psi).
The rear brake pressure should be 52.7 – 64.7 kg/cm2 (749 - 920 psi) for 105 series and 49.7 – 61.7 kg/cm2 (707 - 877 psi) for 100 series.
 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 turning it.
One turn changes fluid pressure by approximately 0.7 kg/cm2 (10.1 psi)
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.
If in doubt, a reputable brake shop with the necessary gauges and / or testing rollers in the floor will be able to test and adjust if required.
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