Hold your horses valves if they are good quality do not corrode. And Boosey made great valves.
Valves accrete residue that is hard and almost impossible to remove without causing catastrophic damage like sandpapering the valve.
Fortunately I have solved most issues and it involves a home chem clean method that I created myself. It is quick easy and costs almost nothing.
Raid the kitchen cabinet and remove tomato ketchup and some kitchen towel or a jcloth.
remove each valve placing them in order so as to make refitting easy.
Plaster tomato ketchup all over the jcloth or kitchen towel so as to make a poultice. wrap the poultice around the valve body and leave it to do its work for an hour or so even overnight is ok. Avoid getting ketchup on the felts or corks. You can wrap tape around the corks and felts if you wish.
The tomato ketchup cannot damage stainless steel, Monel, Nickel, or Nickel silver. It can however damage brass because the acids in ketchup will attack brass.
The reason for using tomato ketchup is because it is thick and gloopy and stays on the surface of the valve you can even smear it over the valve without the poultice and it will stay put.
The ketchup will not get into the inner ports in the valve but you must clean the valves thoroughly after treatment.
Expect the valves to come up like factory fresh after this treatment. DO NOT USE KETCHUUP ON ANY PART OF THE INSTRUMENT EXCEPT THE OUTSIDE SURFACE OF STANLESS STEEL OR MONEL OR NICKEL VALVES.
Leaks can occur at the mouthpiece if it is loose or at the spit-valves / water keys.
The idea is to place a rubber ball into the bell so it seals and then hold down all the valves and blow into the receiver if you cannot feel or hear a leak and the pressure remains high then there are no leaks. If the pressure reduces as you blow in other words you keep pushing air in, then the first place to examine is the water key corks.
If you dont have a rubber ball then pull the valve slide on the Bb / A valve closest to your mouthpiece then operate the Bb / A valve so the air all goes round the loop you just pulled out. Holding down all the valves while holding a finger over the top tube to seal it will act like the rubber ball and seal the instrument for pressure testing.
If that Bb / A valve does not operate then pull the first valve slide and hold the three valves down and hold your finger over the top tube to seal it and blow into the receiver
Of course do all this after chem cleaning the valves with ketchup.
You have two water keys and almost certainly those are the culprits if there are any leaks.
If you grab a wine bottle cork you can fashion little replacement corks with a knife and some sandpaper. This should replace the existing corks that have deteriorated. You want a couple of disk sized for the metal cup that holds them. The size will be around about 8mm x 4.5 mm. They can vary in size so size them for the cup they will sit in and just cut a bit off and trial fit, and repeat until they sit in the cup. Job done.
If you do it right they will hold themselves in by friction and if they seal the instrument then you can pop down to a tech and have him do a proper job with good corks of uniform consistency for a good long lasting seal.
A couple of words
Boosey valves run sweet and silently but can be unusual with external shrouds around the springs, in such cases they can suffer from the spring rotating at one end while it stays fixed at the other during valve assembly and this can lead to noisy or clanky valves. If the valves do not run silent try, once the valve is assembled, inserting a thin rod into the slot in the shroud where you can see the spring inside it and at the bottom of the spring, and then manually lift the spring then release it, this relieves any twisting that the valve underwent when assembling the valve.
Also an instrument of this age will almost certainly have weakened springs, be aware that this might be the case and if the valves are sluggish and you have cleaned them and oiled them then a visit to a tech is recommended for replacement springs.
Corks and Felts will have deteriorated too so pay attention to those.
I know Boosey equipment and believe me it is thoroughbred and professional and highly reliable and robust. It is a fairly easy bet for me to say this is likely to be a sleeper horn that will sound great I cannot guarantee that however. Even the greats can stumble. I do however believe that a really good tech would probably have some good things to say about this instrument.
The only thing that worries me is the unconventional elements.
For example the horizontal rod at the front is reminiscent of a little known instrument built in England with many innovative features. This also had a horizontal rod very similar the one your instrument carries. As I recall it was fitted to control unwanted resonance.
Boosey also had a reputation for innovation
Resonance is desirable in an instrument but there can be undesirable unwanted resonance of small parts and this resonance can create an unplesant buzzing to the tone. This can appear in sounding the purest of tones.
I would hazard a guess that the presence of this rod controls resonance in exceptionally pure tones that this instrument must therefore be capable of.
The circumstantial evidence therefore is that this should be a beautiful and lyrical instrument otherwise they would not have put so much effort in to improving the tone with this device.
This is substantiated by the Class "A" in the name which defines the instrument as one of the higher class of instrument Boosey made. Silbron also suggest high quality. Boosey created its own metal for the higher class instruments, the lower class were Brass, Brass and Bronze were at one time interchangeable names some Bronzes labelled as such are in fact Brasses.
Silbron is Booseys own mixture of Silver and Bronze and an instrument was called Silbron when some of the parts were made with their SIlbron metal this was reserved for higher quality instruments.
In the old days there were no classifications of Student Intermediate and Professional. Manufacturers simply made what they considered appropriate and usual that meant professional quality without calling it professional
Some manufacturers called their professional instruments 16 medals of honour Boosey and Hawkes both called their professional instruments Class "A" and Boosey additionally used the term Silbron to mark the instruments out as using superior metal I believe they claimed it gave superior resonance and lyrical tones.
Clearly the horizontal rod was used to moderate and control and enhance the resonance.
I know this is a lot of text but it is all relevant to your instrument and there is so much to know about them.
They were always innovating and for example they were the first to manufacturer to form perfect tube by hydraulic pressure call hydroforming where they run an undersized tube into a mould or reversed mandrel and then using hydraulic pressure they inflate the tube to fill the mould. This gave perfectly formed seamless tubes very fast and consistently. This has since been adopted by some of the better manufacturers.
Brian
