Central heating pump airlocking

In recent weeks our central heating has been getting air-locked; now on a d aily basis. Heating starts off well but after about 15 mins we can hear ai r rattling in the pipework and eventually the pump in the airing cupboard a irlocks. We have bled all the rads and the system was totally cleaned, and inhibitor placed last March. We bleed air out of the magnet daily and wit h lots of bleeding at the pump, eventually the system starts up.....Pump is just a couple of years old and the heating gate valve was replaced last ye ar. The pipework to downstairs rads runs through the concrete floor and I a m concerned in case there is a leak here. Am now at a loss as to what nee ds to be done......
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