Can be used for both PARACHUTE & 2ND STAGE ROCKET DEPLOYMENT
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Here's something I've been working on for several months. I'll be adding considerable information to this page.
I'll start with a movie to get your attention and curiouity, (and tease you a little)
This is movie demonstrates the flexibility & expansion ability of the hose I use. In the actual deployment device, I add a middle string to prevent the hose from ballooning (like in the gif movie). Here's a photo of what that looks like. As this inflates, it grips the inside of the neck of the 2nd stage rockets.
On the top is an ADJUSTMENT NUT that turns to adjust the "backpressure" going to the 2nd stage rocket. This is much more reliable than using tire valve cores!! This regulates how fast the 2nd stage rocket fills with pressure. If adjusted too slow, it just simply takes too long, you will need to unscrew the nut to speed up filling. If adjusted too fast, then backpressure will equalise in pressure with the booster too soon, and the 2nd stage would fire while still on the ground. If this happens, then adjust the nutclockwise. The idea is to adjust it so that there is plenty of time to safely fire the rocket, and maintain a differential in pressure between the booster and the 2nd stage rockets. For example, if you pressurise the booster to 100 psi, then the 2nd stage rocket should not exceed 70 or 80 psi. There must be a slight back pressure to keep the hose expanded, which grips the 2nd stage rockets.
There are no metal parts, which make this device "LEGAL" in many water rocket competitions. If the hose ruptures (hasn't happened yet), just slide on a new replacement hose and tie three strings in the proper locations.
The pnumatic timer is mounted on the bottom cut section of a 2 liter bottle in this example. It's connected with a 1/8 IP threaded lamp rod. I CURLED the cut edge for strength. The timer has an ADJUSTMENT NUT also, simular to the DEPLOYMENT DEVICE. It's job it to adjust the amount of air pressure leakage to the booster. 
The adjustment is made with a piece of 3/8" PVC pipe that has a slotted end to TIGHTLY recieve a copper penny. The penny will LOOSELY fit
on the end of the ADJUSTMENT NUT with is also slotted.
This is a top view of the booster rocket, looking into the neck. You can see the the ADJUSTMENT NUT is slotted to fit a penny.
The timer and cut bottom of the bottle are glued inside the upper area inside the booster. This makes a SECOND SMALL AIR CHAMBER inside the booster. The TIMER will regulate how fast this SECOND SMALL AIR CHAMBER will bleed off pressure into the booster after launch, which will regulate when the DEPLOYMENT DEVICE releases the 2nd stage rocket.
This shows the TIMER and the DEPLOYMENT DEVICE both mounted on the booster.
The configuration looks like this. Please excuse the tempory drawing, but it should help you understand the concept anyway.
Below (and later) I will give you detailed instructins on how to cut the proper length of the bottle that holds the parachute.
This is a drawing showing the elements of how I attached the deployment device to the bottle cap.
----more later, much more-------