Henk,
One of the problems in giving a definitive answer to your question is that there isn't enough known about silver soldered brass boilers and of the regulations required in Holland to hydraulic test and give a Certificate for insurance.
In the U.K. if you produced a brass boiler to a tester to be Certified you would require to produce design drawings, design figures of the brass tube from the manufacturer on it's strength and integrity and calculations to prove that your design was safe other wise he would refuse to hydraulic test it and issue a certificate for insurance purposes..
In all of the years in testing Copper boilers I have never come across a new brass boiler made by an amateur boiler maker for testing nor do I know where design calculations for a brass design are available, so in general I would not test it.
I don't know where you live in Holland, city or country, so if you are not concerned about having an insurance certificate to cover any accidents or personal injuries with your boiler go ahead and make it in brass.
Just be careful on soldering as it's already been said that the melting point of brass is near that to silver solder and the heat required you could alter the mechanical property's of the brass and if the gauge is quite light you could in the worst case burn a hole in the tube or change the brass structure to become very brittle which will not be noticable until under pressure when it could all give way.
Don't under estimate 1 bar of pressure, it can do serious damage if you are near a boiler at that pressure when it lets go, there is a You-Tube vid some where of a small brass boiler blowing and it's quite frightening, maybe some body can post the link to it.
Remember that Wilesco, Regner and others who make and sell steam plants buy such large quantities of brass tube that they can specify from the tube makers what gauge and make up of the tube is and have been making them for years.
I would strongly suggest that if you wish to make your own boiler make it in copper which is a very well known and proven material for the amateur boiler maker.
Calculations are available ( see the K.N.Harris book MODEL BOILERS and BOILER MAKING ) as are proven designs for copper boilers and safer in the long term.
Don't let us put you of with your brass boiler but be very careful
George
