Cristiano Ronaldo and Angel Di Maria both hit braces and Gonzalo Higuain was also on target in a first-half goal romp.
But there is still little hope of them retaining their title as they trail Barcelona by 15 points despite the Catalan side losing 3-2 against Real Sociedad on Saturday, their bitter rivals' first league defeat of the season.
They are also seven points off neighbours Atletico Madrid who beat Levante 2-0 earlier Sunday.
"As long as there are points available then you can't throw in the towel but the aim isn't to just win for the league but for the other positions behind and to improve," said Real Madrid assistant coach Aitor Karanka.
"All we do is to try and improve and it looks as though we are on the right lines. Against Valencia we have shown what we can do."
There has been plenty of tension in the Madrid camp following their poor form but there were no repercussions from an argument between Mourinho and Ronaldo after the first leg of the cup.
Furthermore, Iker Casillas was preferred in goal to Adan Garrido who had controversially replaced the Spain international before being suspended.
Still they had important absentees at the back with Pepe, Sergio Ramos and Marcelo all missing and they faced a rejuvenated Valencia side under Ernesto Valverde who had won their previous three league games.
After a cagey opening spell Madrid were suddenly presented with a gift for Higuain which he failed to take advantage of in front of goal.
He was left unmarked but side-footed an Alvaro Arbeloa cross wide from six yards.
Higuain was given the chance to make amends a minute later and this time made no mistake. Again it was a case of poor defending as Di Maria scampered clear and set up Higuain who slotted into the corner.
Valencia attempted to respond but they fell apart midway through the first half as their defence was once again exposed.
Keeper Diego Alves twice denied Sami Khedira and Raphael Varane headed wide from a corner before Di Maria got the second.
Ronaldo teased makeshift right-back Ricardo Costa, standing in for Joao Pereira, and crossed for the Argentine to tuck the ball home from close range.
Valencia were in disarray at the back and Ronaldo split them open again as he burst into the left of the area and beat Alves at his near post.
Ronaldo got his second after 41 minutes when he cracked in an Ozil pass from 12 yard and then Ozil was again the provider as he put Di Maria clear for the fifth still before half-time.
Valencia showed character to come back at Madrid after the restart with Pablo Piatti hitting the crossbar and Adil Rami had a header cleared off the line by Fabio Coentrao but Madrid remained organised and managed to keep a clean sheet.
Atletico Madrid equalled their best ever winning streak at home in La Liga in a memorable season for the club with their win over Levante.
Goals from Adrian Lopez and Jorge Resurrecion ‘Koke' saw Atletico win their 12th straight league game at the Calderon and match the club record dating back to a run between the 1939-40 and 1940-41 seasons.
But the win was overshadowed by a thigh injury suffered by 18-goal top-scorer Radamel Falcao which could sideline the Colombian striker for three weeks.