I'm trying to implement a double action piston using SweptVolumes but I obtain an over-constrained system with 2 equations too much. The code for the double action piston is as follows:

model DoubleActionPiston

  // mechanical connection
  Modelica.Mechanics.Translational.Interfaces.Flange_b flange annotation(
    Placement(visible = true, transformation(origin = {104, 0}, extent = {{-10, -10}, {10, 10}}, rotation = 0), iconTransformation(origin = {104, 0}, extent = {{-10, -10}, {10, 10}}, rotation = 0)));

  //hydraulic connection to piston 1
  Modelica.Fluid.Interfaces.FluidPort_a port_1(redeclare package Medium = Modelica.Media.Incompressible.Examples.Glycol47) "feed piston 1" annotation(
    Placement(visible = true, transformation(origin = {-104, 40}, extent = {{-10, -10}, {10, 10}}, rotation = 0), iconTransformation(origin = {-104, 40}, extent = {{-10, -10}, {10, 10}}, rotation = 0)));

  //hydralic connection to piston 2
  Modelica.Fluid.Interfaces.FluidPort_b port_2(redeclare package Medium = Modelica.Media.Incompressible.Examples.Glycol47) "feed piston 2" annotation(
    Placement(visible = true, transformation(origin = {-104, -40}, extent = {{-10, -10}, {10, 10}}, rotation = 0), iconTransformation(origin = {-104, -40}, extent = {{-10, -10}, {10, 10}}, rotation = 0)));

  //piston parameters
  parameter Real L = 0.055 "piston stroke [m]";
  parameter Real D = 0.040 "piston diameter [m]";
  parameter Real d1 = 0.000 "piston 1 shaft diameter [m]"; // there is no shaft on that side
  parameter Real d2 = 0.010 "piston 2 shaft diameter [m]";
  parameter Real s0 = 0.0275 "piston (2) starting position [m]";
  parameter Real c1 = 0.01 "piston 1 clearance [m]";
  parameter Real c2 = 0.01 "piston 2 clearance [m]";

  //pistons modeled as Modelica.Fluid.Machines.SweptVolume
  Modelica.Fluid.Machines.SweptVolume p1(redeclare package Medium = Modelica.Media.Incompressible.Examples.Glycol47, clearance = p1.pistonCrossArea * c1, nPorts = 1, pistonCrossArea = Modelica.Constants.pi * (D ^ 2 - d1 ^ 2) / 4, s(each fixed = true, each start = L - s0)) annotation(
    Placement(visible = true, transformation(origin = {-40, 40}, extent = {{-10, -10}, {10, 10}}, rotation = -90)));

  Modelica.Fluid.Machines.SweptVolume p2(redeclare package Medium = Modelica.Media.Incompressible.Examples.Glycol47, clearance = p2.pistonCrossArea * c2, nPorts = 1, pistonCrossArea = Modelica.Constants.pi * (D ^ 2 - d2 ^ 2), s(each fixed = true, each start = s0)) annotation(
    Placement(visible = true, transformation(origin = {40, 40}, extent = {{-10, 10}, {10, -10}}, rotation = -90)));

  inner Modelica.Fluid.System system annotation(
    Placement(visible = true, transformation(origin = {-74, 70}, extent = {{-10, -10}, {10, 10}}, rotation = 0)));

equation
  //hydraulic connections to the pistons
  connect(port_2, p2.ports[1]) annotation( Line(points = {{-104, -40}, {80, -40}, {80, 38}, {50, 38}}));
  connect(port_1, p1.ports[1]) annotation(Line(points = {{-104, 40}, {-74, 40}, {-74, 42}, {-50, 42}}));


  //pistons do not move independently
  p1.flange.s = L - p2.flange.s;
 
  //piston mechanical output
  flange.s = p2.flange.s;
  flange.f = p1.flange.f - p2.flange.f;
 
  annotation(
    uses(Modelica(version = "3.2.2")));
end DoubleActionPiston;

As can be inferred from the code the double action piston is implemented as 2 SweptVolume pistons/cylinders, "p1" and "p2", with equations linking their position as well as the force and position at the piston shaft is  collected in "flange".

As mentioned I get an over constrained system with:

"""
Check of DoubleActionPiston completed successfully.
Class DoubleActionPiston has 116 equation(s) and 114 variable(s).
62 of these are trivial equation(s).
"""

What are the two variables that are missing or which 2 equations are too much ?