constantly breaking trap seal

[--tom]

A bedroom, basin trap, the highest fixture on the SVP, is constantly breaking it's seal. For example: I set the plug in the basin and covered it with 2" of water, about three minutes later the plug popped up and drained the water while releasing sewer vapours.
Curiously, the basin overflow doesn't seem to be releasing sewer gases.
Any answers?

 

[kevplumb]

are you sure the vents not blocked

 

[--tom]

No, from the ground, the vent, with it's cage, is open.
It is,however, set in a recess in the roof and does not protrude above the roof line. The roof is an unusual 1970's configuration.

 

[JerryD]

Sounds like the sewer gasses are building up and 'burping' back through the easiest route, which happens to be this basin. The soil stack must be blocked and needs venting to the outside air, through the roof somehow. Depending on your configuration, this should be quite easy to do.

 

[--tom]

I meant, seen from the ground the vent is open at the outlet.
However, where the trap waste arm leaves the bottle trap to where it drops is a distance of 300mm then a drop of 400mm then a 90 into the SVP.
So, in effect, it's an S-trap. But this doesn't explain the delay after draining the basin and the bubbling up of the trap seal. Or does it?

 

[Bahco]

Fit an anti vac trap.
This allows air in breaking the syphon effect