Hello, I'm looking for help with rebuilding the petcock on my 80 DT175. I bought a rebuild kit (image 1) but upon taking it apart, I find this rubber sleeve type piece (image 2) which looks too be where the leak was coming from, and that piece was not in the rebuild kit and I can't seem to find it to buy online anywhere. Do i need to get a new petcock completely? Or where can I purchase a rubber sleeve like the one in the image? TIA!!

You need to post images via a 3rd party host, such as Imgur.

You'll need to use a third party like Imgur or Google to upload pics and then link them here.

Not sure what kind of rebuild kit you've found, but it doesn't surprise me that it doesn't fit as there are dozens and dozens of petcock variations available for that model.  

For example, Yamaha sells the petcock rebuild parts BUT they fit the original petcock that came on the bike in 1980 and NOT the new petcock Yamaha currently sells to replace the original.  

(And the last time I looked, the Yamaha repair parts cost as much or more than a complete replacement petcock from Yamaha!)

So if you want to stay original and you have the original petcock, seals should be available at your favorite Yamaha dealer or OEM parts house.

If not, there are many choices for a new petcock that will fit the threaded bung...  for example ebay or someplace like Yambits should have plenty of them.