A tandem master cylinder includes a pair of separately moulded cylinder portions (lA, lB) of plastics material, joined together in co-axial relationship and each containing a pressure chamber (2, 3). Pistons (4, l0) are slidable respectively in the cylinder portions (lA, lB). The cylinder portions are surrounded respectively by cylindrical parts (l5, 2l) joined to their associated cylinder portions at locations outside the limits of inward travel along the cylinder of seals (4A, l0A) carried by the pistons (4, l0). Sealing between the cylinder portions (lA, lB) is effected around facing surfaces of the parts (l5, 2l).