further minor adjustment to steam brake

Author: peternewell

Source/Orts.Simulation/Simulation/RollingStocks/SubSystems/Brakes/MSTS/VacuumSinglePipe.cs

@@ -345,7 +345,9 @@ public override void Update(float elapsedClockSeconds)
                         if (SteamBrakingCurrentFraction < SteamBrakeDesiredFraction) // Brake application, increase steam brake pressure to max value as appropriate
                         {
 
-                            SteamBrakingCurrentFraction += elapsedClockSeconds * lead.EngineBrakeController.ApplyRatePSIpS / conversionFactor;
+                            var diff = elapsedClockSeconds * lead.EngineBrakeController.ApplyRatePSIpS / conversionFactor;
+
+                            SteamBrakingCurrentFraction += diff;
                             if (SteamBrakingCurrentFraction > 1.0f)
                             {
                                 SteamBrakingCurrentFraction = 1.0f;
@@ -354,7 +356,10 @@ public override void Update(float elapsedClockSeconds)
                         }
                         else if (SteamBrakingCurrentFraction > SteamBrakeDesiredFraction) // Brake release, decrease steam brake pressure to min value as appropriate
                         {
-                            SteamBrakingCurrentFraction -= elapsedClockSeconds * lead.EngineBrakeController.ReleaseRatePSIpS / conversionFactor;
+
+                            var diff = elapsedClockSeconds * lead.EngineBrakeController.ReleaseRatePSIpS / conversionFactor;
+
+                            SteamBrakingCurrentFraction -= diff;
 
                             if (SteamBrakingCurrentFraction < 0)
                             {
@@ -376,6 +381,7 @@ public override void Update(float elapsedClockSeconds)
                             float equivalentBrakeLine3PressurePSI = equivalentEngineBrakePipeFraction * (MinimumVacuumPressureValue - MaximumVacuumPressureValue) + MaximumVacuumPressureValue;
 
                             lead.BrakeSystem.BrakeLine3PressurePSI = equivalentBrakeLine3PressurePSI; // If engine brake on, then don't allow engine brake pressure to drop when reducing train brake pressure
+
                             EngineBrakePipeFraction = SteamBrakingCurrentFraction;
                             Car.PreviousSteamBrakeCylinderPressurePSI = 0; // set to zero so that this loop is not executed again until train brake is activated
                         }