How long to wait - coolant bleed

I appreciate that this topic has been covered probably more times than enough.
However I’ ve got a bit of a problem.
Filled it up with coolant and under static conditions I had a drip from the rad vent and from the heater vent. I knew the blleed by the cylinder head was clear as I had had the screw out.
Started the engine and apart from a fuel leak after changing the fuel filter it was leak tight.
With the engine running and temp climbing I could not get any liquid from any of the three bleed screws. Eventually the engine bay fan cut in but not the radiator fan. So it was shut down and left for about 15 to 20 minutes before re starting.

Everything was warming up but still no water from the bleed screws. Then I did something I regret I undid the rad screw some more and it came out with water gushing out (too hot to touch but certainly not near normal temps). The bleed screw is now lost and gone for good into one of the cavities in the chassis below the rad and never to be seen again. Getting a replacement is just one problem. 
But what I want to know is why I can’t get water out of any of the three bleed screws.
Any ideas please?
Its a TF 135 built Dec 2004.
I have just completed cam belt change, water pump replacement, oil filter change, air filter change and fuel filter change, and under floor water rail replacement.

It's just an airlock.
When you get a new radiator bleed screw fill the header tank again. If water doesn't run out of the rad simply get a short length of hose, remove the rad plug and blow down the hose.
You will hear it bubble up through the big underfloor hoses in to the header tank.
I cleared mine that way.

Thank you for that suggestion. Is yours fitted with the PRT a or stat by the head?
I’m thinking it might be something associated with the PRT.
But I’m puzzled why nothing is being pushed out of any of the three bleed points. I’m sure the small one by the cylinder head should be “bleeding” from there. 
Water does appear to be recycling into the header tank. And the level does rise as that tank as it warms up.
 

https://www.mikesatur.co.uk/-MGF-MGTF-Radiator-Bleed-Valve

I fitted on of these.  Bit extravagant  but if to have to replace it anyway?

The threaded hole behind the bleed screw can get grotted up. A good poke with a bike spoke & at a stroke a spurt will rid the dirt.
On the radiator bleed point; it’s a fairly standard size & can be bought for a few quid from a plumbing supplier. I’ve gone back through my orders to see if I can verify the thread form, sadly nothing has turned up.
I remember it was cheaper than the MG original, it was stainless & at that time, by an amount.
M

If we are discussing the bleed in the actual radiator (black plastic thing)  If that is what you have lost, I still have the original.  You are welcome to that if you wish.  (I think I know where it is?)

RE. the airlocks.  I bought a cheapo device for single handed brake bleeding - it was pretty useless at that BUT you could use it to suck water (and air) out of the bleeds.

Found it! this is the ebay description for a replacement bungerup.
M12 x 1.25mm Steel Drain Plug Blanking with Recessed Rubber Sealing Washer
They're going to be about £3.50 at a guess, more if there's carriage, mind you Rimmers don't have stock, which is remarkable & so its got to be good value at several times the price.
M

Thanks all for the help and information regarding the rad bleed screw. I’ll solve that after a bit of panic seeing that Rimmers have them as NLA I ordered one from MGOC . I’m in a bit of a race against the clock with the MOT a due in a few days so it was “any port in a storm” in order to move forward.

Many of you will have had cars “pre” and “post” PRT. I am trying to locate drawings indicating the differences in flow and the consequences of that to see if that helps to produce an explanation.
I think that the PRT stat has to open before any flow gets to the rad.
I think that the heater should get flow almost immediately but need to check this.
I thought that flow should come from the bleed screw on the water rail by the cylinder head almost immediately and if correct why is that not happening?
Why did the engine bay fan come on when nothing would come out of any of the three open vents (rad,heater,water rail). Water was circulating back into the header tank.
Any ideas please?
 

If you want to work on it this weekend the thread is the same as a spark plug so you could use one of those pro tem!

Chap who did work on my car raised the back up - maybe that helps? 

After viewing   courtesy of MG-Rover forum and trevtherevs/dert I am still puzzled as to why fluid is not being pumped out when the vent plugs are opened.
”We” know the rad was full when the plug was lost. It is understandable that water might not be being returned until the PRT opens but coolant should still flow out of that vent.
There is fluid in the rail by the cylinder head and definitely not restriction there but nothing gets pushed  out when the engine was running and water circulation into head tank was taking place.
Similarly the heater bleed point.
It is understandable that in closed vents that water might not circulate due to an air lock but any open vent must surely release water/air until all air is expelled.
Nothing will happen now until the replacement plug arrives other than re fitting the front valance after failing to retrieve the lost plug.
But any theory as to why no water is coming out of the vents will be well received.

If you were around when i was posting about the cooling system tribulations i had on a Mk1 F you will have seen I had not too dissimilar symptoms which defied my own fault finding and had begun to empty the well of ideas on this and the MGR forum. I flushed the system through multiple times with mains water pressure, replaced the thermostat, the water pump and the radiator with no change.

In the end I went back to first principles, looked at a diagram of the cooling system yet again and this time went to the hottest part of the system which is the outlet assembly at the front side of the engine. removing all of it's hoses I was surprised to see the metal spigot i had just removed a half inch hose from was solidly blocked with detritus. This had clearly built up over the years around the tempertaure sensors (being the narrowest flow point in the flow there) blocking the spigot and preventing the bypass flow. Now due to the slightly unusual cooling system configuration the thermostat relies on a working bypass flow to open. Clearing this blockage resolved the issue immediately.