I have an old shower with a single stem mixer. Brand is unknown. The head is leaking, which I am sure is because of the washer at the base of the stem. Well, reasonably sure. I removed the handle and face plate. The stem itself is brass with two square and two rounded sides, but there is no visible washer, just two "ears" on the top and bottom inside of the stem collar. There is a brass outside collar that has a clip (to restrict the stem turning, I guess), which I have removed. Do I try to pull the stem straight out? Even though I have access, it does not pull straight out with a standard wrench. Any ideas? Sorry I cannot upload a pic at this point.

Thanks.

By " The head is leaking" do you mean the part where the pipe comes out of the wall and sprays down on you, a shower head?