Another cause of a leaking retro mixer tap may be a damaged seal or drain gasket. Depending on the cause, the gasket or seal may have eroded or worn. If the valves are loose, the washers may need to be replaced. If the seal isn't loose, you might need to tighten the coupling nuts. If these measures don't help, call a plumber. If the problem persists, you might need to replace the entire unit or the sink itself.