40mm waste block vacum?

[kev the spark]

hi, i replaced bath and fitted new shower tray with 40mm waste to outside of house under the floor, through the brick wall and right angle down to existing 62mm cast pipe(surface), in cold weather the drain backsup, but if i release the reducer clamp on 62mm side(rubber with clamps on 40 and 62 side)can hear air enter and the water flows, it works fine all other times only in cold weather.
any suggestions are appreciated

 

[blup]

The drop is creating a vacuum which sucks up the foul water, the vacuum is broken when you loosen the collar, as it draws air in and breaks the vacuum, or prevents it forming. Try a proper trap bend on the shower waste