Thanks for all of your comments.
With respect to fitting a brake bias valve, I think this is easier on the earlier cars because they don't have a split rear circuit. Also, I haven't seen any bias valves for dual circuit systems and don't really want to change the brake circuit layout (I'm looking to spend as little as possible too).
My understanding of how the proportional brake valve works is as follows (please feel free to correct me if this is wrong):
1) When there is no pressure (brakes not applied), the valve returns to the minimum load position regardless of how much weight is over the rear axle.
2) When the brakes are applied the master cylinder pressurises the rear brake circuit and the fluid passes through the valve, sending fluid to the cylinders and pushing the valve plunger down at the same time.
3) The plunger movement is dependent on axle weight. The lighter the car, the more movement available.
4) As the plunger moves down, the volume inside the proportional valve increases, which in turn reduces the volume of fluid sent to the rear cylinders. Therefore the lighter the vehicle, the more the plunger moves downward, the greater the volume of fluid inside the proportional valve and the less amount fluid is sent to the rear cylinder (preventing locking up)
5) The plunger movement is eventually 'stopped' as it comes into contact with the arm that is connected to the panhard rod.
6) When the plunger is stopped the volume of fluid inside the valve can no longer increase and therefore it is sent directly to the rear cylinders.
Now, what I am thinking is to produce a small bracket which sits underneath the plunger with a screw running through it. When the brakes are applied and the plunger drops, it would come into contact with the head of the screw. I could adjust the screw up and down to simulate different vehicle loads. I could even run a cable adjuster (like for brake balance bars) to the screw and adjust its position from the driver's seat.
Has anybody had a go at this as I believe it would be a very cheap solution to purchasing and fitting a brake bias valve.