You'll need a "tank" to stablize this.
What happens is that the density of the fluid is a function of pressure and temperature. The source will do work to move the fluid, and that work will heat up the fluid. Higher temperature, lower density, so the fluid will try to expand, but it can't because the volume of the whole system is fixed. As a result, pressure goes up.
In fact, depending on if you are using a flow rate source or a pressure source, you may get too high an overall pressure or too low.
The solution is to add a tank component (like this one). Usually these closed loop systems have one. It'll allow the fluid to expand a bit without brining the pressure to a crazy level, as in a real system. If you don't have access to Simscape Fluids, you can try to build a "tank" using the Translational Mechanical Converter (TL) block.
