See Figure 1
The purpose of the deceleration valve is to prevent backfiring in the exhaust system during deceleration. The normal position of the valve is closed. When deceleration causes a sudden vacuum increase in the vacuum signal lines, the pressure differential on the diaphragm will overcome the closing force of the spring, opening the valve and bleeding air into the intake manifold.
Air trapped in the chamber above the vacuum diaphragm will bleed at a calibrated rate through the delay valve portion of the integral check and delay valve, reducing the vacuum acting on the diaphragm. When the vacuum load on the diaphragm and the spring load equalize, the valve assembly will close, shutting off the air flow into the intake manifold.
The check valve portion of the check and delay valve provides quick balancing of chamber pressure when a sudden decrease in vacuum is caused by acceleration rather than deceleration.