U can leak them down as long as your in 50 degree weather or warmer.
I've leaked down motors at room temp and it doenst vary much

I will however warm mine up this tues.
I have been told I have a few low cyl in my new shortblock,My TAVIA leakdown testetr will tell me if that true...

I'm shooting for 4 to 8% leakdown....
Its a new motor....

 

I also did my leakdown while my motor was cold and I got between 4-8% on the good cylinders and then I get the 90 % blowbuy on the 1st bad one which is where I quit.

 

JS thanks I didn't know 50 was the critical temp.

 

yeah, we have sub 40* temps here right now...