Terios rear brakes - proportioning valve?



I really hope you can help me - my Terios has
failed its MOT due to poor rear brakes Sad
Its a 1999 model.

Apparently the rear brake force is about 1000 (dont know what units Scratch one-s head ) compared to 1700 at front. Last year rear brake force was OK at 1700.

The handbrake is fine. The rear cylinders look clean and so do the shoes. We are wondering if there is a fault with the proportioning valve or maybe the ABS unit (no fault light though).

I would really appreciate any ideas you guys might have - I am fearing something expensive here Sad

Help please - thanks, terioskat