What is this?

[Dougal9887]

It's on the rear brake pipe tee junction.
Dougal.

[phillpot]

It's a pressure regulator valve. Its purpose is to reduce force at rear brakes under heavy braking, preventing rear wheel lock up.

[TJC46]

Dougal9887 said:

It's on the rear brake pipe tee junction.
Dougal.

Brake modulator. A bit like a brake bias valve. It moderates the pressure to the rear brakes.

At this price i would just clean yours up ! Its either £60 or £80 depends on which one.

https://tvr-parts.com/tvr-parts/part-details/tvr-j...

[Dougal9887]

Thanks for the replies.
It's not an easy thing to test or indeed know if it's actually working.
However as I imagine it's just a spring and a valve it likely to be OK.
Dougal.