I am sorry to say, that this style of prop building does not really ever provide satisfactory results. Luckily with the amount of air your losing, the PVC never really pressurises and explodes. You should never pressurize PVC pipe with air pressure.
It would be best to build the prop using metal, a real pneumatic cylinder, and a solenoid valve that is designed for use with air pressure.
If you are not comfortable with designing and building your self, there are alot of options out there for premade kits, and devices. Here is also a good how to for building one http://www.instructables.com/id/Tras...-Kit-and-MAKE/
Here is also a good how to for building one http://www.instructables.com/id/Tras...-Kit-and-MAKE/