One thing to keep in mind about the pressure rating of the radiator cap you are using.  All are rated relative to a standard sea level ambient pressure.  This means that he air pressure itself is contributing 14.7 psi at sea level. 

Everything Ed said plus more whining from Hanover.

 

In systems where the pressure control cap is mounted on the radiator top, or on a tube, The system pressure builds to the cap pressure and the cap leaks for a second or so, and system pressure drops a bit and the cap seals again. This goes on until no more expansion occurs, and the system remains at cap pressure until shut down.

 

So long as the system pressure is higher than the local pressure in the make up bottle (catch can) no coolant can be recovered by the system. No coolant will return to the system until the system temperature is very low, and system pressure drops below that in the catch can, which can be a long time after shut down. This system works fine, (same as millions of old cars) except that it takes several heat cycles to expel all of the air from the coolant. 

 

Most modern cars have very small radiators, and low coolant volumes. Coolant costs lots of money. These system are not happy with even the smallest amount of air. So, they use a pressurized make up bottle, just like all Mazda's had back in the 70s. Mazda's are unhappy with even the smallest amount of air in the coolant.

 

In the picture there is a swirl pot. I stole that from the Cosworth engine installation manual. For racing engines that cost more than your house. They want the air out of the system in one lap or less. So if you don't have a swirl pot, don't bother installing the engine. That is what Cosworth said not me.

 

The pressure cap is on the make up bottle, the cap on the swirl pot is just for filling, and it seals solid.

In this (stock Mazda) system as the coolant changes volume even slightly the coolant flows freely into and out of the make up bottle. The air space above the coolant is at the cap pressure, and the cap only vents air when the relief pressure is attained. When throttle is pulled off, system volume goes down and coolant flows from the bottle to the engine, but not air. Any air that goes through the swirl pot pops to the top and any volume increase carries the air to the bottom of the bottle where it pops to the top.

 

So usually in one heat cycle the air is gone. This system requires only that you keep the bottle 1/3 full of coolant when cold. Advantages are that it works perfectly. Mazda put it on millions of rotaries. You can break open the system with the engine hot (engine change at the track) by opening the lever cap as only air pressure is vented. It provides emergency coolant should a leak develop. Not much but it may turn out that just another 100 yards of power can make a big difference. The system is not complex, and if you have a good high spot to vent air to the bottle, even less complex.

 

The pressure bottles can be made of expended fire extinguisher bottles (aluminum) and are available in aluminum from racing supply houses. The make up bottle can be mounted anywhere, and without regard for elevation. It can be mounted low on the firewall. Mine is mounted on the passenger side floor. The filler neck determines what model of Stant lever caps can be used. Also available from racing supply houses.

 

Lynn E. Hanover