Intermittent brake failure

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..

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.

Is the Brake Pedal going to the floor or, you have enough Pedal but the brakes just don't stop the vehicle?

Don't know about the Dodge, But that vintage Ford is famous for calipers sticking

Have you ever changed the brake fluid? If not it's high time.

KD4UPL wrote:
Have you ever changed the brake fluid? If not it's high time.

AMEN!

How old is the brake fluid, and was the "failure" after using the brakes either hard or for a long time (like on a steep descent)?

KD4UPL wrote:
Have you ever changed the brake fluid? If not it's high time.

That's very likely the culprit.

Brake fluid absorbs moisture from the air.

The more moisture it absorbs, the lower its boiling point becomes.

When it boils, it is no longer an incompressible liquid - so your brake pedal is no longer capable of applying any pressure on it.

Also, the boiling point of fluids decreases with altitude. The higher you go, the easier it becomes for the fluid to boil.

Finally, if the brake fluid has ever been previously replaced, it's possible that the replacement fluid has a lower boiling point than the manufacturer's engineers intended that vehicle to have (e.g., used DOT3 replacement fluid in a vehicle designed to use DOT4 fluid):



(The "dry" boiling points shown on the chart are for brand-new brake fluid, and the "wet" boiling points are for fluid that has had a chance to absorb some moisture.)

Thank you for all your thoughts. The brake problem is so random. It only happens once every few years. Nothing obvious like brake fade. I think I will start by changing the brake fluid and then work my way towards the master cylinder.

I would check the vacuum pressure system too. 20 year old hoses tend to get minute cracks.

If my life depended on it, which it does, I would change the brake fluid AND the master cylinder and take a long hard look at all the hoses or just go ahead and change those also. Why wait for the situation to appear every so often try to diagnose a life threatening problem?