Alpha cb24 boiler drain cock doesnt exist?

Hi, Can one of you peeps please confirm if its !Ok! to 'Drain Boiler' Via (Primary Pressure switch)
whilst renewing it when task is completed? i have a a ''Funnel and Hose Attached'' to direct water to a large container. i have considered (Draining Via ''Pressure Relief Valve'') but to renew this Valve the small retaining screw on the side of the valve is to dodgy, cant even get a grub screwdriver in to unscrew, limited access, and ive been advised its best not to drain via this valve. So is it ok ok to Drain Via (Primary Pressure switch) with no complications.the reason i ask is my Alpha cb 24 combi boiler hasnt got a drain cock on the boiler, and i have a new Diverter Valve to replace. asap thanx rob