Which valve do you have?

This pool product site has them displayed nicely: http://www.poolplaza.com/C-V-BW-PF1.html .

Here's a photo of a typical side mount multiport:


You may be able to repair it by
(1) removing the lid
(2) compressing the spring so the handle is unloaded
(3) remove pin #18
(4) disassemble
(5) replace O rings #23
(6) reassemble and reinstall.

Of course, if sand has gotten into the stem, and scratched the sealing surfaces, O-rings won't fix things.