A fuel injection pump is provided with timing control in which a hollow cap removably mounted in a bore of the fuel pump forms a pressure chamber in which the advance piston is axially movable for controlling the timing of injection. An annulus around the cap provides a passage for the delivery of pressurized fuel to the pressure chamber and a flat reed valve in the pressure chamber overlying the inlet port of the pressure chamber isolates the ring seals for sealing the annulus from the repetitive high pressure pulses imposed when the pumping plungers move past the cam lobes during pumping strokes which otherwise may cause the seals to be extruded into the clearance gap between the cap and the bore.