I have done something similar to what you are doing.
I used a small deposit, I believe they call them accumulators.
They are only small sized tanks of aluminum or other materials, you place them near the valve:
___
COMPRESSOR=======(ACC)==[VALVE]=>>> SYSTEM...
I have some experience with this.
Although some may think that unscrewing is better, there is nothing faster than stripping parts off.
There are several factors that you have to take into acount:
1.- Thickness of walls
2.- Diameter of part (the more the better)
3.- Material (softer is better)...
Why don't you check this article,it may help.
http://www.immnet.com/articlelibrary/archive/getOneArticle.php3?getArtID=1664
It's in the IMM Injection molding magazine
To have a correct transmision of light you should not forget to treat your plastic part as an optical fibre.
The principle of transmision is that light should "bounce inside of the plastic.
Optical fibres have to 2 layers of different densities to achieve refraction, your second layer will...
You may want to use the parallel port with a dll from visualbasic called "inpout".
Just do a search for it, I used it some time ago to control stepper motors from the parallel port and also for controlling a pneumatic valve.
