Sources close to the situation tell us ... the valet, who claimed the judge actually spit on him twice during a heated confrontation, will not press charges against Judge Mathis.

The Detroit Police Department tells TMZ ... just after last month's incident at Flood's Bar and Grille, police contacted the valet and asked if he wanted to press charges, and the valet told cops he needed to consult his attorney.

Since then, we're told police have made several attempts to follow up with the valet, but have not been able to make contact ... and, without cooperation from the valet, police can't make a case against Judge Mathis.

We obtained security cam footage of the incident, which shows Judge Mathis berating the valet as he sits in a Jeep. Mathis lunges his head forward into the car window -- the moment the valet claims the judge hocked a loogie right in his grill.

Judge Mathis denies spitting on the valet, who he claims kept him waiting outside the restaurant for 40 minutes. The guy accidentally left Mathis' keys in his pocket while he was parking another car.

The valet clamming up is an abrupt change of course ... as we reported, he initially wanted a DNA test to prove Mathis spit on him, filed a police report for assault and his legal team claimed there were at least 2 witnesses who can back up his story.