why does this stop the bubbling?

[welshrx-7]

hey guys i got bubbling colent when i knock the car off, but when i squeeze the pipe that comes off the top of the ast it stops. is the ast cap knackered? confuses me a bit.

[CharlieP]

Hiya- I had some bubbling noises, I changed the AST cap and solved the problem.
 I\'ve also heard that ASTs dont last forever, so might be advisable to replace that too!