How can I improve the free up and down mechanism for my weight sorting machine?

[dsesmg]

Hi, I am a hobbiest and I am trying to build a weight sorting machine that sorts out weight object according to their weight.
So, basically, there are carriers that are used for transporting weight object and I attached a guilde piston mechanism underneath the carrier so that the carrier could freely position itself on a loadcell(weight sensor) enabling the microcontroller to detect the most accurate weight readings as possible.
The design of the carrier is this.
http://www.flickr.com/photos/34955071@N02/3918540769/sizes/l/
And this is the actual carriers attached to sorting conveyor.
http://www.flickr.com/photos/34955071@N02/3736310225/sizes/l/
See that white things, that's the carriers.
The guide piston which attached to carrier looks like this.
http://www.flickr.com/photos/34955071@N02/4028566051/sizes/l/
And the guide piston mechanism is like this.
http://www.flickr.com/photos/34955071@N02/4028566073/sizes/l/
http://www.flickr.com/photos/34955071@N02/4028566079/sizes/l/
http://www.flickr.com/photos/34955071@N02/4028566081/sizes/l/

Here is the problem.
The weight of the carriers all vary because the guide piston mechanism I came up with has significant friction, causing the bad readings.
The guide piston will move only up and down. but the metal angle(yellow thing) in the picture makes friction with the PVC plate which is attached horizontally to guide piston(This prevents the guide piston not to rotate while attached to the carrier, otherwise, the carriers would spin during the conveyor moving)
If I increase the gap between the metal angle and the PVC plate, the carrier will tilt and this causes the carrier to contact with adjacent carrier. The guide piston also makes fricktion internally inside the cylinder. This friction varies.
I couldn't think of any way to simplify free up and down motion of the guide piston. Also, considering the cost aspect, the current guide piston mechanism was good. but considering the performance, it is bad.
Could please somebody advise me how to achieve the most smoothe and efficient method to make the carrier up and down freely?

 

[FredGarvin]

I think I see what you have going on but there is one thing that isn't quite clear. What relation to the carrier does the grey PVC plate have? Does the carrier rest on the PVC plate to get the measurement?

 

[dsesmg]

Hi, the grey PVC plat is used to prevent the carrier not to spin during the conveyor moving. The piston assembled in the cylinder spins as there is no stopping mechanism in the cylinder. This causes the PVC to make contact with the metal angle. So, I better off eliminating this PVC plate anyway. Then, how can I stop the carrier from spinning...