My Graff vintage 4120 kitchen faucet has low hot water pressure but high cold water pressure. How can I increase the hot water side?

It is likely debris caught in the cartridge. Flush your water heaterm remove the cartridge with the main shut off. Cover the valve body with a towel and have someone else turn the main on for a moment and then off. This will clear the line, you have to cleanclear the cartridge too. Contemporary Bathroom Designs | GRAFF Where are you and which model