I am working on a 93 Cobra. The proportioning valve is missing the rubber plug at the forward end. I do not want to have to gut the PV and convert over to manual. I was able to find a rebuild kit for the internals but this kit does not come with a new plug for the end cap. What are the options here. I have a few spare PV laying around but sadly they all seem to have the same plug missing or damaged. What is the function of this plug? Is it a tattle-tale for the seal going bad internally?