The visitors had the opportunity to establish an 18-point gap over arch-rivals Real Madrid following the capital club's shock loss at Granada on Saturday, but turned in a performance well below their usual high standards as Valencia were unfortunate not to take the win.
Ever Banega opened the scoring with the first shot of the match after 33 minutes, but Valencia's lead lasted just five minutes as Lionel Messi fired home from the penalty spot after Pedro had gone to ground in the area under the challenge of Joao Pereira.
Valencia came closest to winning the match in the second half but Valdes produced two saves out of the top drawer to first deny Sergio Canales and then the dangerous Roberto Soldado.
The draw saw Barca establish a 12-point lead over second-placed Atletico Madrid ahead of their late kick-off against Real Betis.
The hosts went ahead with the first shot on target for either side.
Barcelona's defence was at fault for the breakthrough just after the half-hour as Gerard Piquet's clearing header from Soldado's cross went only as far as Banega, who drilled home from 16 yards.
The visitors did not take long to draw level, however, after Pedro hit the deck on the edge of the area after Pereira rushed in with a clumsy challenge.
The responsibility from 12 yards fell to Messi, who stepped up to slam home his 45th goal of the season - and in doing so extend his record scoring streak in La Liga to 12 matches.
Many at the Mestalla would have expected Barca to take the game by the scruff of the neck after pulling level, but instead it was the hosts who made most of the running in the second half.
Soldado muscled in front of Javier Mascherano to head wide in the 51st minute in an early warning shot for Valdes, who would soon be called on to take centre stage.
Barca's best chance of the half fell to Cesc Fabregas as he got in behind his marker and ran in on goal only to fire against the outside of the post when he should have at least hit the target.
Soldado blazed wastefully over moments later as another chance went begging for Valencia, and Los Che were frustrated again in the 68th minute when Canales struck a fine curling effort from the angle of the 18-yard box that Valdes was more than equal to, flying to his left to make a great save.
But better was to come from Valdes in the final minute as he first got down bravely to deny Canales as he tried to connect with Aly Cissokho's centre, before somehow scrambling to his feet to fling himself in front of Soldado's close-range follow-up and preserve a valuable point for the league leaders.