my punto keeps loosing coolant?

i have had an ongoing saga for the past couple of months, my car started over heating once every 2 -3 weeks, when the temp gage rose i would switch off the engine and go check unde4r the bonnet to find the water escaping from the lid of the water bottle. I tok it to a garrage who pressure tested it and could not find any leeks but told me than i had an airlock which could force the water through the cap. The car was fine after this for about 2 months until it happened again. my uncle used caustic soda to clean any blockages and resulted in my radiator badly leaking he uses the stuff on his own car but dont know if he used too much but he reckons that the soda just exposed a weakness,after having my radiator replaced we found another leak which turned out to be a corroded core plug behind the gear box.Since then i am still loosing water but not as fast but i topped it up the other day and now i think i have another airloch. i know exactly where the valves are but would like if someone could give me some step by step instructions.Also would like to know if anyone has any ideas for the root of the problem as i know the gasket can be the culprit but my oil is black, no white smoke from exhaust and my heater works just fine except in the past when the water was pouring out from the cap. Any help wouyld be much appreciated.