Would a failed thermostat (closed position) cause back pressure on the cooling system and force all the coolant out of the plastic resevoir bottle when hot?

 

Yes it will overheat too which in turn will force the water of the easiest escape location.

 

could be several things.
failed stat, if it is closed the car will overheat.
weak spring in the cap letting the pressure escape
head gasket leaking gasses into the coolant - pressure build up will force coolant out via the filler cap or even pop the rad if it is old.

 

But in my experience, thermostats fail open.....

Could also be headgaskets.

 

Its ok the stat is fine. I drained all the coolant last night and remove the rad hoses and the stat. Put it in boiling water and it operated normally. 2 things though. One of the previous owners has had a leak at sometime and has lazily but some type of sealer in the system as there were fine copper particales in the coolant. I found the original source of the disappearing coolant (The rad has what appears to be a leak somewhere) The sealer they put in the system has obviously worn off now and the rad has started to leak again. Second thing at some time the thermostat must have been replaced as there was only one, yes one bolt holding the housing on!!! For the price of a rad and how easy they are to change I dont know why people do these quick fixes and think they are permanent solutions!!!!

 

Oh dear, I am guessing the fine copper particles are K-seal - this is not a good indication.
http://www.kalimex.co.uk/our-products/k-seal
This is used to cure leaks and in most cases h/g failure/head warping as the copper is drawn to the leak area and seals it - i know this because i used it with success on my old Pug 406. The car was only worth around £500.00 so was not worth spending money on getting the h/g done so K-seal did the job.

I would hazard an educated guess the rad failure is a symptom - read on; My symptoms where a leaking rad which blew (due to the additional pressure of combustion gasses in the coolant and being old and tired) I initially thought it was the rad only, replaced the rad - then the only place the pressure could escape was via the pressure cap and the coolant pissing out of the header overflow when stationary after a run or when i slowed/stationary in traffic.

K-seal worked but is not a guarantee or a permanent fix.

Get a sniff test done to rule out h/g

This is probably not what you want to hear, i hope i am wrong but your story sounds all too familiar.

 

Could also be a failed water pump.....