Had the same problem years ago when converting my FC with standard radiator to a recovery system.
I found,in my case, that the depth of the opening for the cap in the top tank fller neck was deeper than the depth ( or reach ) of the radiator cap from screw up face of the cap to the end seal on the cap so to put it a bit simpler form, the cap end seal,when fitted,was not sealing hard under spring pressure against the seal face of the top tank neck opening.
What was actually happening was the radiator/cooling system wasn't even being pressurised at all and pressure was "leaking" to the outside via the overflow pipe as the end seal was being held just off the seal face in the neck.
From memory the depth of the HR top tank neck isn't as deep as a grey motor one so if you are using the same cap as you had on a HR then the above mentioned problem would happen.The cap end seal would not reach the seal face in the grey top tank neck seal face and would be held about an 1/8th inch os so from the seal face.
Also get a set of vernier calipers and measure to ensure the upper seal of the radiator cap isn't also blocking the overflow pipe opening in the top tank neck when the cap is fitted and that the upper seal is wide enough diameter wise to give effective sealing in the neck as you need a good seal for the vacuum created when the motor cools to draw the coolant back into the tank from the overflow bottle.
You will have to do some measurements with the verniers to get the measurements that you need to match up to a better fitting cap if that is the problem.
Just because a cap will fit in the neck doesn't mean that it will work, you will have to do some measurements to ensure the seals are positioned to effect the overflow on engine run and drawback into the tank on engine cool.
I had my FC in early/mid '80's and from memory the cap I ended up using was a Commodore cap,probably VB,VC,VH ?? ( or maybe WB ) but not 100% on that.Also vague recollection that I had to also modify the locking tabs on the cap as they were too long to engage in the opening for the tab in the filler neck.
This may or may not be your problem but it may help.
Cheers,
Terry.
And also getting back to Franks suggestion above
yes, definately check your hose to overflow pipe connection to ensure it is sealing well and the condition of the radiator overflow hose as well.As ,as Frank said, check that the end of the overflow bottle is held about 1/2 to 1 inch above the bottom of the overflow tank and there is sufficient coolant in there as well.
Check also that the top filler lid in the recovery bottle has a small hole in it and isn't blocked as it needs a hole in it ( small one ) to also effect the filling and drawback of coolant in the bottle.
Recovery systems are lazy!! They would rather suck air than suck water ( its easier to suck air than water ) and if there is a hole anywhere in the overflow circuit or bad connection it will suck air rather than water.
