An ultrahigh-pressure fluid system, such as an ultrahigh-pressure fluid pump, includes an improved dynamic seal assembly that can withstand pressures above 40,000 psi, and more preferably, pressures up to and beyond 87,000 psi. The dynamic sealing assembly includes a plastic seal having a bore through which the plunger reciprocates, and a bearing positioned adjacent the seal, also having a bore through which the plunger reciprocates. A seal carrier surrounds the circumference of the bearing and is subjected to a compressive force that is sufficiently high to circumferentially collapse the seal carrier in a radial direction against the bearing. This collapse of the seal carrier against the bearing causes an inner surface of the bore through the bearing to achieve substantially uniform contact with an outer surface of the plunger when the assembly is subjected to ultrahigh-pressure, thereby eliminating gaps that occur in prior art systems. By eliminating any gap between the seal carrier and the bearing, and between the bearing and the plunger, the present invention eliminates any pathway through which the seal might otherwise extrude, particularly when subjected to high pressures up to and beyond 40,000 psi, and more particularly, up to and beyond 87,000 psi.