When fitting new guttering always test it with water whilst fitting it. If its a 3 sided property. Fit one side then test it with a bucket of water to make sure you have a decent enough gradient, so the water will drain away to the down pipe. Once you have achieved this move onto fitting the second side with new guttering. Then test it again with water. By doing this you are ensuring the gutters will work for many years to come. The only time it will falter is when it becomes blocked up. Then this will be the responsibility of the owner to keep up with the maintenance. Every ones guttering will become blocked at some point and the only sure way of making sure they continue to work for many years to come is having them cleaned out. I personally do not recommend brushes for the guttering because they automatically take up all the space up in the gutters. All leaves , debris will gather on top of the brushes giving the illusion that they are blocked, plus eventually everything on top of the brushes will decompose and drop through the brushes blocking the brush and the guttering. Waist of time in my opinion. I have attached a video of me testing the guttering as I fit one side.