I've got a 650 Thunder series carb. The instructions say the front nipple is for the PCV valve. There is a port out the back side for power brakes. If no power brake port is needed, you are supposed to cap off the port with a supplied plug. The instructions say NOT to hook the PCV valve to the rear port. My question is why?

Isn't manifold vacuum the same no matter what port you use? I want to run an oil vapor separator mounted on the PS inner fender right next to the firewall. It would be a much cleaner install to run the line to the rear of the carb. I do not have power brakes.

I'd either call Edelbrock or turn the carb over & see if there is any differences in the routing/dia of the two passages other than the front barb is a made to order fitting for a PCV hose to slip on it

Edelbrock said you can physically hook up the line to the rear port but you get a cleaner burn of the gasses with it hooked up to the front. I'll have to look at the bottom of the carb and see how the ports are tied in to the base plate.

Is it just a matter of there being constant flow past the front port since that's the primary side?

I'd call again & see what another tech has to say . or maybe even call Holley & as them that as a generic question. Other than that I'd eyeball the bottom & see what that tells me then in your case I'd maybe go ahead & plumb it to the rear & see how it acts. It'd be easy to switch if it was wonky

Here's my The front hook up goes straight into the bores a mixes better this is in case there is a little oil or vapors being sucked thru the pcv valve and hose. The back side hook up is for vacuum booster and it doesn't suck any oil or vapors thru so they can run that up a little ways on the body and channel it thru to the bores. If you run the pcv on that back side the channel it could possibly clog from sucking in oil/vapors. Make sense?

Yes, makes sense. I'm going to run an oil separator. Hopefully that will filter out most of the oil anyway. Thanks guys.