1994 Peugeot 306 engine keeps pressurizing

hi there, iv'e got a peugeot 306 1.9 d turbo just changed the cylinder head and gasket, thermostat, water pump and radiator also flushed out the heater matrix but now the radiator keeps emptying itself all over the floor. When i bleed the system its ok but when i put the bleed nipples back in the radiator empties !!! any ideas please