Wow... that had to be one of the scariest "OH $#!%!" moments he ever had... I'm sorry to hear of his passing.
Gotta be careful when working with brakes... They are your life... It's the one system I've always hated working on, regardless of the size of the vehicle. Hydraulic brakes actually scare me much more than air systems... I've had hydraulics fail on me, but fortunately never had an accident because of it.
I remember driving an old bookmobile I had purchased back home... The brakes were iffy to begin with and were hydraulic. The wheel cylinders on all four were really old, and leaking, but I didn't know this. I tested things before driving, and everything worked fine... I took my time, drove slowly and made sure I had lots of room in front of me and escape routes if something went wrong.. That plan was wise.
Halfway home, I went to stop at a stop light... Hit the brakes, they engaged... Then all of a sudden the pedal went soft... The rear brakes had just failed due to a ruptured line and wheel cylinder on the right rear. I pulled in to the walmart we were next to and found the wheel completely covered in brake fluid. Because it was a dual chamber brake system, the front brakes still worked just fine. Fortunately, it was only an 80 mile trip... Broke down 40 miles from home and had to leave it for the time being.
I came back with with a new cylinder, repaired it and finished the trip. The brakes on that bus are all brand new now, as is the vacuum assist... I still have that bus and it's now sitting in a field as a storage unit. heh... It could be driven and used still, but I decided it just wasn't worth continuing the repairs and retired it permanently. When I clean it out, I'll probably end up selling it to one of the local hunters for them to pull/drive into their hunting spot as a cabin.
But yeah... Brake systems scare the hell out of me...