Intermittent brake failure

I would start with a complete fluid flush with a pressure bleeder. Second would be to replace the master cylinder.
Maybe just do both at the same time.

By failure I assume the peddle went to the floor without the expected braking action.

Back to basics.

Pull the calipers and brake pads, inspect the slide surfaces of the pad ears.

The pad ears must glide across the slide surfaces with only light friction, often times the pad ears are slightly oversized and are hard to move by hand. Add in a little rust and the pads stick and freeze in the caliper. Depending on where they stick you can get no brakes or brakes work on and off or badly stuck they will not release from the rotor. I find I have to grind down the pad ears some to get them to slide easily by hand.

Inspect the caliper pins, this is a known part that often sticks, caliper pins should easily move in and out by hand, can get replacement pins at your local auto parts store.

Inspect the caliper hoses for kinks or damage, sadly these hoses often get damaged when mechanics fail to support the caliper and let it dangle in mid air when servicing the brakes. This causes the hose to kink internally which is not noticeable from the outside. Sometimes the hose deteriorates on the inside and collapses enough to block flow. For vehicles the age you are talking about, replacing the hoses may be in order just because of age.

Baring hose or physical pad/caliper you could have calipers that are way past due replacement time, some caliper cylinders use a plastic piston which over time deteriorates and gets stuck intermittently.

If all of those checks out fine, then perhaps your master cylinder may need replaced..