Place a flat-head screwdriver into the adjustment screw, and turn it clockwise to raise the fill valve which will allow more water to fill in, or lower the fill valve by turning the screw counter-clockwise if your tank needs less water. Flush the toilet after these adjustments and see if the water fills properly.Click to see full answer. Beside this, why would a toilet bowl not fill up?The most common reason your toilet bowl water level is low is a damaged fill tube. The fill tube is a small flexible plastic hose. When this happens, the toilet tank fills with water and the valve shuts off the water flow before the bowl fills adequately.Additionally, how do you adjust a toilet fill valve? To adjust this type of valve, you simply turn an adjustment screw located on the top of the valve. To raise the water level, turn the adjustment screw clockwise; to lower the water level, turn the screw counterclockwise. The water level must be below the top of the tank’s overflow tube. Accordingly, how do you know if your fill valve is bad? 3 Ways to Tell if Your Have a Faulty Fill Valve 1 – Toilet Runs Constantly. This is a common signal that a toilet’s fill valve is broken. 2 – Toilet Won’t Flush or the Flush is Weak. A weak flush or no flush at all when the handle is depressed may mean that the toilet tank is not being filled with water properly. 3 – It Takes a Long Time For the Tank to Refill. Can I manually fill toilet tank? Flushing a Toilet Manually If you do not have running water, you can still keep your toilet functioning by filling up the tank manually. Or, if you have a swimming pool you can borrow water from there. You will need to fill the back of the tank for every flush.