low air pressure on breaking system recuring?

I bought my lf 45 daff and drove it home, parked it overnight. The following day i started it and gave it time to build the air for breaks. When i drove it for 300meters i applied hard breaks and insteady the break pedal started going down without vehicle stopping and altimately a sign appeared on the dash board indicating low air pressure followed by an alarm. I stopped and air built up to the bar 10 but when i tested it again it repeatedly showed that it was still loosing pressure. I am worried that i inherited a big problem what could that be possible causes and how i can amend/repair the problem.