Not sure about the C30, and not sure if you are talking about the bleed valve either. The bleed valve is basically a hole in the compressor casing, about 1 1/2" across, with a flat disc that can close it off, and the disc is on a housing with a spring pulling it open. You can push the disc closed with your fingers, and wedge it shut for a compressor wash, to make sure the soapy mix goes down the spout and doesn't squirt out over the engine compartment.
On a C20, the bleed valve is spring-loaded to the open position. At startup, it allows excess air to vent overboard so the compressor can spin up easily, and there is not too much air going into the combustion chamber, to prevent a stall/surge.
As the engine accelerates, air pressure from further down the compressor pushes the valve slowly closed, until at about 92% N1 the valve is fully closed and all the air goes into the fire. The valve doesn't allow sucking, it only lets the air go one way, out. It can open when a sudden acceleration is needed, to reduce the back pressure.