Without knowing what is broken, we can't tell you how to fix it.
The Seals are shot. Without knowing the model I can't tell you how to fix it. I suggest that you go online and search for "Airgun Repair" and find a shop that can fix it for you.
YOU MUST buy new ones.....if your brakes are shot they are shot.
How to fix the error on FMC
Take it to an authorized repair center
buy a new screen.........................
It all depends on the model and what is wrong with it.
Flush it : that will fix it!
You must specify the model and what is not working.
You can get all of the work done before fixing it. This makes it easier to see the mistakes as a whole.
Although their website mentions that they fix model trains, there is nothing to indicate that they fix electronics. Their link is attached.
You can use tape for a temporary fixing or glue for a more permanent fix.
Build and fix model is the worst model. It does not involve any steps for SDLC and is highly expensive.