Bit desperate here.
I've just attached my air compressor to one of the tyres on my sister's car(because it was low). When I took it off, all the air started escaping, as if the valve has got stuck. I put the dust cap back on, and this seems to have stopped the sound of air escaping, although I'm sure it must still be escaping but more slowly.
I then did the same with another tyre with the same result.
What should I do, as I fear by morning, both tyres will be flat, and she's coming to collect it in half an hour.