for 1 thing,you should not be charging to 2.5 bar, 1 bar is normal. system pressure will increase, as it gets hot, the relief valve operates at 3 bar. as you have it at 2.5 a lot of pressure is going out, when it operates, thats is probably why it drops to 0.5.
Turning the valve should not do anything except release pressure, But it is not the correct way, they can leak afterwards. Why did he turn it anyway, was it leaking? as turning it a few times can flush the valve seat and stop it from leaking.
SD, February 2012
