Broken valve?

[Gixxerman]

My rear tyre lost some pressure so I decided to pump it up using my Halfrauds mini-pump.
This is the first time I have used this pump and it took a bit of experimentation to get it converted from schrader to presta.
Even then it took me some time and a lot of fiddling about to get some air into the tyre. I eventually managed it.
Went into the garage the next day and tyre was completely flat. Strange I thought, must have done something wrong with the valve as it is the first time I have used a presta valve.
Pumped it up again. Next day completely flat again. This was repeated the next day with the same result.
I know that I do not have a p* as it has been up for months.
On closer examination of the valve, it looks like my ham-fisted attempts have bent the top of the valve slightly (the bit that sticks up with the captive nut on it). I suspect that this is the problem.
Some questions:-
1) Could this be the cause of the tyre not staying up?
2) If the valve is goosed, can I fix it in situ? If not, is it easy to change?
3) How do I prevent it happening again? The method I used was to screw the nut all the way to the top then attached the pump. Is this correct?
If it is the correct way of doing it, then it is very difficult to hold the pump steady enough so that the valve is not damaged during the pumping action. What is the correct way to inflate a presta valved tyre?

 

[rogersavery]

take the tube out, pump some air in it, dip it in a bowl of water and see where the air is coming out, if it is the valve then bin the tube, if you have a puncture then patch it

 

[02GF74]

you need to rule out a puncture but first read the bit below.

the presta valve should stay closed by air pressure - the little threaded bit with the small nut is there to screw it closed so the valve cannot open due to centrifugal force.

Now you have bent the little threaded part, I suspect when you screw the nut down and or/ fit the cap, the valvey bit inside that we cannot see is getting skewed and is releasing air.

I have successfully managed to straighted these out - use a pair of pliers.

If it breaks off, and that has happened to me , well the tube is probably suspect.

 

[Gixxerman]

Well thanks for the advice.
I checked the tube and I indeed have a p*, albeit a very slow one. Felt inside the tyre where the p* was and it was a thorn that done the deed. Rubbing it with my finger broke off the offending thorn point.
This is very strange as it was pretty much keeping up for months and it is only very recently that I have pump up my tyres for the first time since getting the bike 4 months ago. The tyres went a bit low on pressure over that time, but not too bad so I didn't bother to pump them up over that time (yes I know flame me )
It was only after I pumped up the tyre that it went flat overnight.
Very weird. I can only assume that the tyre had gone down enough so that the tube had come away from the thorn and so wasn't p*. As soon as I pumped the tyres back up from their normal pressure, the tube expanded and pressed against the thorn and gave me the p*.

 

[andrew_s]

Thorns can do a pretty good job of plugging the hole they've just made, so you only start getting a leak when you do something that disturbs the relative position of tyre and tube. I had one that held air at about 85psi for a couple of months - if I pumped it up to 100, it went back down to 85 overnight, then stopped.