Yeah, that sounds like the brake bias valve.
It looked like it was going to be a pain to fit, so I got rid of it and stuck a mondeo valve in the rear line instead.
As it happens I failed IVA because my rear brakes were far too weak
I took the Mondeo valve out and replaced it with a bit of brake line and passed.
The rear brakes on the MX5 are a bit pants at the best of times, which is why I guess a bias valve isn't needed.