located in a central portion of the fluid supply chamber 31 a the cap 32 includes a cylinder 32 b that is a cylindrical blind hole in a central portion of the fluid storage chamber 32 a a piston 34 is slidably housed inside the cylinder 32 b a coil spring 35 is compressively arranged inside a fluid chamber 32 c including the piston 34 and the cylinder 32 b such that the piston 34 is pressed toward the diaphragm 33 this pressing force presses the piston 34 against the diaphragm 33 when the diaphragm 33 is in a neutral position the protruding portion 31 b and the diaphragm 33 face each other via a gap t 2 the oil feed line 4 is in communication with the fluid storage chamber 32 a through a channel 32 d formed in the cap 32 the oil feed line 4 is in communication with the fluid supply chamber 31 a through a channel 31 d formed in the variable throttle base 31 and the channel 32 d the discharge port 31 c is in communication with the hydrostatic pocket 2 a through an inflow passage 2 b in the table 2 details of the piston 34 will be described based on fig4 the piston 34 has a small diameter portion 34 b and large diameter portions 34 a arranged at opposite ends of the small.