Any idea why my radiator hoses would suck flat? Happens right when I start the car, hoses slowly return to normal after a minute or so after shut-off.

Bad hose. Theres supposed to be a spring in the hose to prevent it from colapsing due to the suction from the water pump.

I dont think thats the problem. I went through 2 sets of hoses already. With the stock radiator they sucked flat, I replaced em and they were ok. My current setup uses a Ron Davis aluminum radiator. The lower hose is made up of a small rubber hose that joins the radiator to a section of exhaust pipe, then another section of hose to join that pipe to the water pump. The upper hose hasnt changed at all since it was working with the stock radiator.

I dont really understand why the lower hose is routed to the exhaust. Make sure the little button valve on your radiator cap is functioning. If its not your radiator will turn into a vacuum when you turn your car off.

Haha, the hose isnt routed to the exahust, I just have a section of exhaust pipe for a piece of the lower radiator hose. I'll check the cap today, maybe the little pressure release handle is stuck or something.