The valve(s) found on a brass-wind instrument that are opened to release saliva accumulated while playing.

when an unusually tall girl is giving a man a rusty trombone in the 69 position while pissing in his mouth

When receiving a rusty trombone the man flatulates on the hoes tongue.

emptying out the spit valve
-verb

A. The wringing out of the testicles during the course of a rusty trombone.

B. The drinking of accumulated sweat, saliva, and/or anal discharge wrung out from the testicles during the course of a rusty trombone.

A Woman who cannot seem to swallow semen. She spits it out.