Reservoir cap, function - GF20.30-P-1000-01A
The cap (2) has two opening pressures.
1st stage
If the pressure in the cooling system exceeds the first compression stage, the valve opens and allows the pressure to escape into the upper part of the cap (2)
2nd stage
If the pressure continues to rise, the pressure relief valve of the 2nd stage (4) opens and allows the pressure to escape through several ventilation bores (a) mounted at the cap of the expansion reservoir (1), along the annular port (b) into the outlet nipple (c) and a hose extension to atmosphere.
The 2nd stage cap (2) eliminates the need for the overflow tank.