This is a pressure bleeder reservoir cap for the brake fluid circuits on all Fiat 124, 128, 131.
Use for brakes and clutch on X1/9 and Lancia Scorpion/Montecarlo cars.
Fill your reservoir with fluid, and use a low pressure air source such as a bicycle pump to push fluid (and any trapped air) through the system and out the bleeders.
A much more effective method than vacuum pump bleeding tools, or stroking the pedal alone in an attempt to fill an empty system.
Once the circuit is filled and good pedal pressure is attained, perform a final bleed using the "pump-and-hold" method.
Extremely effective for hydraulic clutch bleeding on X1/9 and Lancia Scorpion/Montecarlo.