If you have an open system (feed & expansion cistern in the roof) the pump may be pulling air down the vent pipe if the vent pipe is incorrectly positioned, or there may be a leak in the pipework on the negative pressure side of the pump (through valve stems ect). It may not be air at all, if there is no inhibitor in the system it could be hydrogen but I doubt you would have to bleed the rads twice a week if this is the case.
A auto bleed valve like this can be installed in a higher part of the system that is getting air. Mount the valve upright. The Watts model FV-4-1/8 or FV-4-1/4. I would prefer the use of the smaller one 1/8 may take a little longer to bleed but it is being used a maintenance bleeder and size shouldn't be a issue.