Reddit reviews Control Devices NC Series Brass Non-Code Safety Valve, 25-200 psi Adjustable Pressure Range, 1/4" Male NPT
We found 3 Reddit comments about Control Devices NC Series Brass Non-Code Safety Valve, 25-200 psi Adjustable Pressure Range, 1/4" Male NPT. Here are the top ones, ranked by their Reddit score.
Relief pressure adjustable from 25 to 200 psi1/4" male NPT threaded inlet connectionBrass construction and stainless steel spring for corrosion resistance250 degrees F maximum temperature for use in high temperature applicationsSilicon O-ring provides leak-proof seal to within 10 percent of set pressure
This is the one I went with. https://www.amazon.com/gp/product/B007GDXT3O/ref=ppx_yo_dt_b_asin_title_o05__o00_s00?ie=UTF8&psc=1
I don't know what the purpose of an s30 valve is (maybe its a UK thing given the terminology used) but would a spunding valve fill the same task?
Mine was cheap to build off of this valve with a new weaker spring:
https://www.amazon.com/Control-Devices-Non-Code-Adjustable-Pressure/dp/B007GDXT3O/ref=mp_s_a_1_4?keywords=adjustable+pressure+relief&qid=1568799629&sr=8-4
Why a radiator cap? I wouldn't think you'd need pressure relief unless you're doing something really crazy to power the water out. Most just use a bike pump and some regular ABS fittings, like this: https://www.youtube.com/watch?v=NodTvFcK0Mc
And you can buy low pressure relief valves with standard pipe fittings, like these at Amazon: https://www.amazon.com/Control-Devices-Non-Code-Adjustable-Pressure/dp/B007GDXT3O/ref=pd_lpo_sbs_328_t_2?_encoding=UTF8&psc=1&refRID=BMYQPGS9H0XS6WN8V3QX
Otherwise, if you've say got the cap already, why not just buy a radiator neck? your auto parts place probably has them. Rather more of a pain to fit into ABS I suspect, but with enough glue anything is possible.
Seems like a lot of work, but a fun project.