Over time (our sheds are about 20 years old) these small plastic parts start to perish and become brittle. I would really recommend getting some new shed door glides if you are having any trouble with sliding shed doors. My old neighbors just asked me recently, to help with a well-seasoned metal garden shed – the same problem, sliding door HW broken.
For our SPACEMAKER shed, the part(s) are called DOOR REPLACEMENT GLIDE KIT (item # 5116-778), and should be coming in soon (Aug.2015). A little for the installation: the metal C-shaped track in our shed is not accessible from either side.
Eventually they will break and the shed door will no longer be connected to the door tracks.

It is not worth fighting with the shed doors when it is quite a simple process to change the glides!
Are customized made to suit the house you've there are a lot of plans for wooden storage vitally. Well in fairness one of the main reasons that we put off changing our shed door glides was because we could not find any replacements.
The Treco (Australian) website also includes printable instructions and a very useful video that shows how to install the new shed door glides which I have included below.
Once that happens it becomes very annoying having to awkwardly shuffle the door aside every time you want to open the shed! I changed the door glides over in our two garden sheds recently and found that it was a fairly simple process.

Getting the doors lined up with the new glides, so they could be reattached was probably the hardest bit, but after doing a couple (two for each door) it got easier.
The shed door glides are the small moulded plastic parts that connect the doors to the track at the top.

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