The intermediate layer has a yield strength that is less than that of the aluminum-based bearing layer.

The bladder is a tubular member which has a section (91) of weakened elastic strength relative to the remainder of the member.

Said pipe has no more than 15 inclusions/100 mm2 (grain size: 50 µm or greater), and has a yield strength of no more than 400 MPa.

The gauge section is. differentially heat treated from the ends so that the gauge section has a lower yield strength than the ends.

The outlet end (18) of the second chamber is defined by the inner surface (40) of a sleeve (42), the material of which has a yield strength less than that of the materials of the mixing head body and the second plunger (32).