Looking on the internet deeply has found these results:

let you know later is the most popular phrase on the web.

let you know again 178,500 results on the web	let you know later 1138,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
I'll let you know when she starts speaking to me again. Wish I could let you go again, but you know... I'll let you know if she strikes again. I was just calling to let you know everything's fine before you called again. No, but if I need my house burned down again, I'll let you know. I'm calling to let you know Deva is not in school again this morning. I'll let you know if we need to talk again. You know they'll never let you see him again.	I'll let you know later. Can I let you know later? I'll let you know later. Let you know later today. We recorded you and we'll let you know later. I'll let you know later. I'll let you know later. I'll let you know later.

let you know again

178,500 results on the web

let you know later

1138,000 results on the web

More popular!

I'll let you know when she starts speaking to me again.
Wish I could let you go again, but you know...
I'll let you know if she strikes again.
I was just calling to let you know everything's fine before you called again.
No, but if I need my house burned down again, I'll let you know.
I'm calling to let you know Deva is not in school again this morning.
I'll let you know if we need to talk again.
You know they'll never let you see him again.