Engine brilliant but cant get cooling system to pressurise

Hi Gents
As some will know I recently rebuilt 4.6 engine and had new liner put in heads skimmed all back together and running very smooth indeed the only issue is that after doing all this replaced the orings as there was a weep I have the lpg vapouriser just other side of bulkhead so they get knackered easily anyway temp is sitting absolutely dead centre have done about 60 miles so far with some in traffic some on motorway I cant get the cooling system to pressurise there is no loss of coolant I have put a new cap on expansion tank any ideas
Ta

The usual problem is too much pressure. Is it getting up to temperature? Pressure cap duff if it is.

Got to say that is my conclusion as well it gets up to temp as usual heater works etc etc and gauge stays rock solid so must be duff cap letting by - will get another one and see what happens .
Its having first real trip this weekend off to Hesdin in Pas De Calais
Thanks for input once again

Mines the same, top hose is never firm. Been like it since I've had it nearly 2 years and 10k. Runs fine. Wouldn't worry. Like the man says to much pressure is worse.