Abstract: A cover for a retention chamber in a master cylinder reservoir which will permit a probe to pass through a tab and pressurize a piston in a pressurizing chamber with a test fluid under pressure. This test fluid will allow an operator to visually inspect the master cylinder for fluid failures at various pressures. In the normal operation, the tab of the cover will allow fluid to flow from a supply chamber into the retention chamber. However, during a brake application when fluid is transmitted through the compensator port as the pressurizing piston moves past the compensator port, the tab will prevent this fluid from being squirted into the supply reservoir. This will assure that a minimum quantity of fluid is always present in the retention chamber to operate the brake system even though a low fluid level indicator in the supply chamber presents a failure warning signal to the operator.