Gravity rainwater tank won't fill cistern
I have a Fluidmaster (400UK) ballcock in my cistern. I want to use a rainwater tank to fill it just using gravity. The tank's 3 or 4 feet higher but the water won't run in. I've read in this question (https://www.askmehelpdesk.com/plumbing/toilet-cistern-slow-fill-350059.html) details on how to replace the washer, and I've taken mine apart. But there seems to be a very small pinhole for the water inlet. If I make the hole in the washer bigger then it won't shut off when the float rises? Any ideas? Thanks