I had ASE heavy and light brake licenses. I had something like 16 of those ASE licensees. I let all that stuff go when I retired. I was a mechanic for 30-years in a phone company fleet, when we still had some single cylinder master cylinders, even on the great big F800 trucks were still single systems. We had at times close to 400 vehicles and we kept them at least 15 years. We changed the fluid every two-years. We serviced them every 3-months but then after years they switch us to six-months regardless of the mileage to save money. Every service we had to pull the drums off; we pulled back the wheel cylinder boots to see if the pistons were starting to leak. We always kept a look at the rearend of the master cylinders, when they got old, they start to leak a little down the boosters. when they started doing this it was time for a rebuild, we were way ahead of any total failures in the brake systems. In 30-years we never had a brake system failure. I've always had complete faith in single brake systems if I maintained them. The problem is they sit around; the fluid gets moisture in them; it leaks out and pretty soon you have an empty master cylinder when you step on the pedal. If you just maintain them the way you were supposed to, they are perfectly safe to use. Watch the fluid level in the master cylinder, watch for leaks in the wheels cylinders and change all the fluid every two years. If your rubber pistons are old getting replace them. One hint is the fluid will turn "black" from little pieces of the rubber pistons. The fluid should always look clear, otherwise replace all the rubber pistons and flush out the fluid. We used to keep a stock of all the rubber pistons rather than replace wheel cylinders we rebuilt them. Even back then we were getting bad new cylinders, and we found it better to just rebuild the ones on the vehicles. One of the things you really had to watch for was the gasket on lids of the master cylinder, otherwise the fluid could splash out of them and empty the reservoir. There is nothing wrong with a single system, you just need to maintain it.
I been retired now for 15 years. I have about a dozen collector cars, and I have gotten so bad about checking the brake fluid. I've had a small stroke. I will need to rebuild the hydraulics in all on them if I even feel like driving them again. Except the '29 Model A with the mechanical brakes. That one I can let sit for years and the pedal is always good.
