Brake lag?

Yep.

 

That’s the nature of air brakes, one reason why you take a test not to have any restrictions on your license.

 

Juice brakes, yes, air brakes, should be no lag, none that I ever encountered. Again, moisture somewhere could do that.

 

I've always noticed just a tiny tiny bit of lag to when the trailer grabs.

 

nope, not normal. should react the same at the first push. That sounds like 100 percent foot valve issue. Put a new one in

 

Air brakes have about 0.3-ish seconds of lag time. Takes time for the air to flow from the pedal to the relay valves and from the relay valve to the chambers. Then the chamber needs to stroke and either actuate the caliper or push on the slack adjuster and rotate the cam to push the shoes into the drum.

 

Also depends on the type of valves and the crack pressures they have. A R12 relay valve has a hair more lag then an R14. Some may have a 1 psi crack some have a 3 psi. Also some antilock systems valves are “lazier” then others.