it's Milwaukee. it's $50-65 each retail(made in USA).
http://www.sureshotsprayer.com/default.htm
first, yours is painted finish or stainless? painted: oil based liquid, stainless: water based liquid.
fill it up with air and dunk it in a kitchen sink just look for bubbles. pretty simple design. if you have the regular hex filler cap uses a regular schrader valve. if you got luck, putting in a new one will solve the problem.
the other kind filler cap with bars, it uses left hand thread schrader valve, it's a ***** to find one.
I just ordered 2 new filler caps, total $24 with shipping.
if you have a Zep cleaning product rep, Zep sells re-badged Sureshot sprayer, they can get you the same thing.