Creo Flow Analysis - Simulate Rate of Increase in Pressure on a Piston

I am using PTC Creo 7.0 and want to analyse the behaviour of pressure in a braking system. I was thinking to use a simplified model consisting of a length of brake line (4.8mm bore) connected to a piston inside a cylinder. (35mm diameter with 0.5mm stroke)

I would then introduce restrictions in the brake line and analyse the effect on pressure build up behind the piston.

Does anyone have experience with a model similar to this?

As a starting point, I have created a transient flow analysis of a simple brake line with a restriction, but without the piston element: the brake line is set with 8000000Pa inlet pressure and 0Pa outlet pressure but this is not yielding the expected results (i.e. a ramp up in pressure from zero, finishing with the outlet pressure equaling the  inlet pressure).

This is the result I obtain currently:

Looks o.k. so far, however if I simulate a number of different restriction diameters, I get this:

I would expect final outlet pressure to equal input pressure once the system reaches equilibrium if I have modelled it correctly - the results make me think my chosen model settings are wrong somewhere.

Inlet and Outlet settings are:

Inlet:

Outlet:

Any ideas where I might be able to improve this?

Thanks very much for any help!