just my 2 cents.
tools are an investment. buy quality once. be proud knowing you have the ability to do the job right.
learn to do things right the first time. once you are proficient in doing things properly then you will get the skill set to know where you can safely cut a corner here and there.
give respect to earn respect.
for some of us, our tools are our livelihood. i dont typicly loan my stuff out to many people. and seeing you spout off like you did, seals the deal as far as your chances. it shows disrespect. if you arnt going to show respect to people you are asking help from, how are we to expect you to respect our property?
take this as a life lesson.
that said. with out doing any actual measurements, theres a few things to consider, as Ben pointed out, theres alot more pressure involved than simple opening pressure. think about how much wall thickness you are removing. if i recall a 19mm hex is about 21mm across the points, so thats atleast 2mm of wall thickness you are removing. now you have un eaqual wall thickness so as it encounters pressure spikes it will flex un evenly eventually causing fatigue. now take into account work hardening causing embrittlement. last but not least, what happens if you screw up even one of them? delivery valve holders arnt just your average parts store item, what is a replacement going to cost you? hmm.... more than the correct socket...