Abstract: A power servo booster in which the diaphragms are freely pivotable from the power piston so as to prevent folding and collapse of the diaphragms in the non-powered state. A shell of the booster is divided into two volumes with a power piston and diaphragm in each volume dividing the corresponding volume into chambers at a negative pressure and at atmospheric pressure. The inner peripheral ends of the two diaphragms are fitted into grooves formed on the sides of the pistons in such a manner as to be freely rotatable or pivotable thereby preventing folding of the diaphragms.