After over 4 years of faithful service, our flattop grill developed a slow leak and it failed to pass the gas inspection.
When we sprayed soap and water on all the joints in the gas lines, we saw nothing bubbling up.
Through process of elimination, we determined that the grease inside the valves had dried up.
You can purchase valve grease online, but we found that brake grease is about the same consistency and is certainly capable of withstanding the heat.
The repair only took about a half hour to complete and you can buy brake grease at any automotive store.