[Bug] Emergency brake doesn't drain the brake pipe completely

[hotdamndel]

On trains with brakes that include a brake pipe, throwing handle to emergency will still keep some pressure left, even though the concept of automatic air brakes universally means venting the brake pipe to atmosphere at emergency setting so the pressure should drop to zero. Currently, it leaves a small inconfigurable amount of pressure (probably not all pressures are calculated relative to atmospheric?) in it, which is physically impossible in reality.

[leezer3]

Basically, pressure is only actually changed if the difference in values between that required and the current is above the tolerance value of 5000pa (a small fraction of atmospheric pressure, let alone brake air pressure)
See the process from the Automatic Air Brake:

OpenBVE/source/TrainManager/Brake/AirBrake/AutomaticAirBrake.cs

Lines 95 to 262 in 1f12f7b

	if (brakePipe.CurrentPressure > equalizingReservoir.CurrentPressure + Tolerance)
	{
	// brake pipe exhaust valve
	double r = Car.baseTrain.Handles.EmergencyBrake.Actual ? brakePipe.EmergencyRate : brakePipe.ServiceRate;
	double d = brakePipe.CurrentPressure - equalizingReservoir.CurrentPressure;
	double m = equalizingReservoir.NormalPressure;
	r = (0.5 + 1.5 * d / m) * r * TimeElapsed;
	if (r > d) r = d;
	brakePipe.CurrentPressure -= r;
	}
	else if (brakePipe.CurrentPressure + Tolerance < equalizingReservoir.CurrentPressure)
	{
	// fill brake pipe from main reservoir
	double r = brakePipe.ChargeRate;
	double d = equalizingReservoir.CurrentPressure - brakePipe.CurrentPressure;
	double m = equalizingReservoir.NormalPressure;
	r = (0.5 + 1.5 * d / m) * r * TimeElapsed;
	if (r > d) r = d;
	d = brakePipe.NormalPressure - brakePipe.CurrentPressure;
	if (r > d) r = d;
	double f = mainReservoir.BrakePipeCoefficient;
	double s = r * f;
	if (s > mainReservoir.CurrentPressure)
	{
	r *= mainReservoir.CurrentPressure / s;
	s = mainReservoir.CurrentPressure;
	}
	brakePipe.CurrentPressure += 0.5 * r;
	mainReservoir.CurrentPressure -= 0.5 * s;
	}
	}
	if (brakePipe.CurrentPressure + Tolerance < auxiliaryReservoir.CurrentPressure)
	{
	if (auxiliaryReservoir.CurrentPressure + Tolerance < brakeCylinder.CurrentPressure)
	{
	// back-flow from brake cylinder to auxillary reservoir
	double u = (brakeCylinder.CurrentPressure - auxiliaryReservoir.CurrentPressure - Tolerance) / Tolerance;
	if (u > 1.0) u = 1.0;
	double f = auxiliaryReservoir.BrakeCylinderCoefficient;
	double r = brakeCylinder.ServiceChargeRate * f;
	double d = brakeCylinder.CurrentPressure - auxiliaryReservoir.CurrentPressure;
	double m = auxiliaryReservoir.MaximumPressure;
	r = GetRate(d * u / m, r * TimeElapsed);
	if (auxiliaryReservoir.CurrentPressure + r > m)
	{
	r = m - auxiliaryReservoir.CurrentPressure;
	}
	if (r > d) r = d;
	double s = r / f;
	if (s > d)
	{
	r *= d / s;
	s = d;
	}
	if (s > brakeCylinder.CurrentPressure)
	{
	r *= brakeCylinder.CurrentPressure / s;
	s = brakeCylinder.CurrentPressure;
	}
	auxiliaryReservoir.CurrentPressure += 0.5 * r;
	brakeCylinder.CurrentPressure -= 0.5 * s;
	}
	else if (auxiliaryReservoir.CurrentPressure > brakeCylinder.CurrentPressure + Tolerance)
	{
	// refill brake cylinder from auxillary reservoir
	double u = (auxiliaryReservoir.CurrentPressure - brakeCylinder.CurrentPressure - Tolerance) / Tolerance;
	if (u > 1.0) u = 1.0;
	double f = auxiliaryReservoir.BrakeCylinderCoefficient;
	double r = brakeCylinder.ServiceChargeRate * f;
	double d = auxiliaryReservoir.CurrentPressure - brakeCylinder.CurrentPressure;
	double m = auxiliaryReservoir.MaximumPressure;
	r = GetRate(d * u / m, r * TimeElapsed);
	if (r > auxiliaryReservoir.CurrentPressure)
	{
	r = auxiliaryReservoir.CurrentPressure;
	}
	if (r > d) r = d;
	double s = r / f;
	if (s > d)
	{
	r *= d / s;
	s = d;
	}
	d = brakeCylinder.EmergencyMaximumPressure - brakeCylinder.CurrentPressure;
	if (s > d)
	{
	r *= d / s;
	s = d;
	}
	auxiliaryReservoir.CurrentPressure -= 0.5 * r;
	brakeCylinder.CurrentPressure += 0.5 * s;
	// as the pressure is now *increasing* stop our decrease sounds
	AirHigh?.Stop();
	Air?.Stop();
	AirZero?.Stop();
	}
	// air sound
	brakeCylinder.SoundPlayedForPressure = brakeCylinder.EmergencyMaximumPressure;
	}
	else if (brakePipe.CurrentPressure > auxiliaryReservoir.CurrentPressure + Tolerance)
	{
	double u = (brakePipe.CurrentPressure - auxiliaryReservoir.CurrentPressure - Tolerance) / Tolerance;
	if (u > 1.0) u = 1.0;
	{
	// refill auxillary reservoir from brake pipe
	double r = auxiliaryReservoir.ChargeRate;
	double d = brakePipe.CurrentPressure - auxiliaryReservoir.CurrentPressure;
	double m = auxiliaryReservoir.MaximumPressure;
	r = GetRate(d * u / m, r * TimeElapsed);
	if (r > brakePipe.CurrentPressure)
	{
	r = brakePipe.CurrentPressure;
	}
	if (r > d) r = d;
	d = auxiliaryReservoir.MaximumPressure - auxiliaryReservoir.CurrentPressure;
	if (r > d) r = d;
	double f = auxiliaryReservoir.BrakePipeCoefficient;
	double s = r / f;
	if (s > brakePipe.CurrentPressure)
	{
	r *= brakePipe.CurrentPressure / s;
	s = brakePipe.CurrentPressure;
	}
	if (s > d)
	{
	r *= d / s;
	s = d;
	}
	auxiliaryReservoir.CurrentPressure += 0.5 * r;
	brakePipe.CurrentPressure -= 0.5 * s;
	}
	{
	// brake cylinder release
	double r = brakeCylinder.ReleaseRate;
	double d = brakeCylinder.CurrentPressure;
	double m = brakeCylinder.EmergencyMaximumPressure;
	r = GetRate(d * u / m, r * TimeElapsed);
	if (r > brakeCylinder.CurrentPressure) r = brakeCylinder.CurrentPressure;
	brakeCylinder.CurrentPressure -= r;
	// air sound
	if (r > 0.0 & brakeCylinder.CurrentPressure < brakeCylinder.SoundPlayedForPressure)
	{
	double p = 0.8 * brakeCylinder.CurrentPressure - 0.2 * brakeCylinder.EmergencyMaximumPressure;
	if (p < 0.0) p = 0.0;
	brakeCylinder.SoundPlayedForPressure = p;
	airSound = p < Tolerance ? AirZero : brakeCylinder.CurrentPressure > m - Tolerance ? AirHigh : Air;
	}
	}
	}
	else
	{
	// air sound
	brakeCylinder.SoundPlayedForPressure = brakeCylinder.EmergencyMaximumPressure;
	}
	double pressureratio = brakeCylinder.CurrentPressure / brakeCylinder.ServiceMaximumPressure;
	deceleration = pressureratio * DecelerationAtServiceMaximumPressure(brakeHandle.Actual, currentSpeed);
	}

I suspect the tolerance value was intended to stop fluttering of gauges, especially at either end of the range.

At present, I think this is probably a 'feature' of the brake system simulation.