Car is a 1995 XJS 4.0 with the Teves III powered system for the rear brakes. I seem to have brake fluid weeping very slightly from the rear brake pressure delay valve located on the RHS inner front wing underneath the power steering reservoir. To test if this was the leak I wrapped the valve with paper towel held tight with lock-ties, put a wadge of paper towel between it and the PS reservoir. drove the car, used the brakes hard and sure enough the paper towel around the valve was now damp. Also I recently replace the power steering hoses and reservoir and cannot detect any fluid leaks from these so I'm pretty sure the pressure delay valve is weeping through the O ring.

I think the fluid is weeping past the O ring (I have checked the brake line connectors for tightness and in my test the area around the O ring was the most damp) but from researching the various threads on this forum I don't think the O ring is a seal, rather a way of stopping contamination getting in through what looks like a tiny vent hole.

Is there any way I can test if the valve is functioning properly? The valve can be split but all that does is reveal a small chamber and no obvious way of getting into the actual mechanism to examine and / or replace seals.

I have a spare valve but without knowing how to test it I'm not sure if fitting it will make my problem better or worse.

Any ideas, thanks.

As a plan B I guess I could replace it with a 3rd party proportioning valve, does anybody have any experience of this?