HA;

If you got the pressure cap with the overflow bottle, you're more than halfway there...THAT will become the pressure limiting device, and the former pressure cap on the radiator just needs to be disabled and simply open to the overflow pipe...I simply did this with an insulated wire stuck into the overflow pipe and partially around the sealing surface to hold the pressure seal open (wire OD is smaller than the ID of overflow pipe, so does not block it).

Like so:


That solution has worked perfectly for me for years, but if it looks a bit too rustic, you could gut to original cap of its spring and pressure seal, and use it as a simple cap (flat cap, cover...whatever you want to call it)...again, pressure limiting will be performed by the pressure cap on the overflow tank...I can't speak to the special cap and one-way valve which James speaks of, but my method uses the OE parts and is completely non-destructive.

Cheers