How to simplify sqrt without imaginary unit

I’ve a system of equations and one solution that Mathematica throws is:

$$ x=frac{mathbb{I} b sqrt{pA – pS}}{sqrt{-3 pA + 3 pS}}$$

knowing that $pS,pA in mathbb{R}$ and allways

$$ pS > pA $$

we can factorize the square root of the numerator like this:

$sqrt{(-1)(pS-pA)}=mathbb{I}sqrt{pS-pA}$

wich turns the solution into:

$$ x=-frac{b sqrt{pS – pA}}{sqrt{-3 pA + 3 pS}}=-frac{b}{sqrt{3}}$$

how can I get that result within Mathematica? I’ve used

expr//ComplexExpand//FullSimplify

without success