my suggestion , would be to find a lawn mower repair shop (that's not too busy out of season) and nicely ask the attendant to make you a new brake , believe it or not , lawn mowers use a lot of similar things that strollers do (wheels, cables,tubing ,Etc.), and chances are they can with a bit of time make you something to work from what they have lying about . as this model is as you said .discontinued and no new parts are available
Old clip just pulls out with pliers if it's jammed. Hard part is finding a new clip. Try eBay or face book. I sell them but have to post them from New Zealand. Google bugaboo repair guy.
You may have a puncture you would find and fix this the same as a bike tyre. Pump it up take out the inner tube find where the air is leaking from (sometimes submerging in water helps as you can see air bubbles coming from hole) then fix with a puncture repair kit