Leaky Water Gauge Valves

I've at last reached the stage of pressure-testing my engine with air. Unfortunately, I'm finding that all three of the water gauge valves leak.
I've connected my air line direct to one of them to make things easier to see. Open, the valve lets through loads of air, as expected. But when I close it, I'm still getting a fair amount of air leaking through.

I followed the assembly instructions carefully: I deburred the cocks; I fitted them into the bodies; screwed in the retaining bushes (they screw in fully onto the body); inserted the seals; and gently tightened the gland nuts. I've dismantled them to check for anything amiss, and remantled them, but still the same situation.
The cocks feel reasonably tight in the PTFE; I can turn them, but there's some resistance. I do note that two of them won't press fully home. I measure the depth to the pre-fitted nut, but the inserts miss this be a couple of millimetres; I can only see a semi-circle through the valve, not a full circle.

Do these behave differently when hot? Is there something I can do to make them seal when cold? Should the inserts go home, and how should I get them to do so?

I hope someone has dealt with this problem and can advise me, please.

Hi Stu
"Do these behave differently when hot?"
In short yes they do get better when hot I had all 3 leak so returned them to STW when they returned they were no better.
Once in steam all the valves settle down the whistle valve is another such valve.
As for the bottoming in the seat the end of the cock will probably have a nipple left behind from machining make sure this is flattened off.

and now having seen this photo I am off to clean my roof SHAMEFUL behavior on my part
Simon C

I fitted a "O" ring between the nut and the valve and that solved the problem

Thanks guys, fingers crossed for when it's hot.
And i have noticed the whistle valve leaking too.