1/2 psi =roughly 14" water or 1" HG (mercury)
You should be reading negative pressure (vacuum) or you'll get leaks.

Is this a turbo? Too much back pressure can create crankcase pressure past the ring type seal in the turbine side. Could even be a turbo problem without excessive back pressure?

Do you have a vacuum gauge? I would check that actual manifold vacuum is acting on the crankcase.

When you blow into the crankcase you should meet resistance, you need to be sure everything is sealed up tight or that small amount of manifold vacuum will be lost.

The drainback hose on the oil separator may be fractured. (mine was) Wouldn't change the vacuum but might alter the function.

I think you should see 6" water vacuum, which can't be measured with a PSI gauge. Don't actually know an exact figure but it must be negative pressure.