What is it?
 

Assessing the condition of my gently neglected purchase and noticed this when I was removing the master Cylinder.
https://cimg8.ibsrv.net/gimg/www.jag...8408ed7dcf.jpg

clear fuel looking filter just sitting there with nothing connected to one end.



Also, what's the best option for a rebuilt.new master? Car had no brakes and master was leaking bad out of the rear.
Kevin

The fuel filter looking thing is the distributor vent. Air is pulled from there, through the distributor cap and into the crankcase. Otherwise there is a possibility that crankcase fumes can come up into the sparky environment of the distributor and cause it to explode. I have seen photos of this happening, so it's more than just a theoretical possibility.

For a master cylinder try any of the forum sponsors. SNG Barratt, Coventry West etc.

I picked up a MC rebuild kit from terrysjaguar. Most of the jag parts places carry them. Usually you'll just need to replace the seals and top ports ( all in the kit I got ). If you find any wear or damage internally you could hone it out, but I'd replace it at that point.

Buy a new M/C, say I. Be safe. The old one will almost certainly be rusty inside. I expect your calipers will need doing/replacing too, as will the three flexibles> Also very carefully check the hard lines for rust, particularly the line to the rear and those on the rear cage.
Greg