Intermittent brake failure

[Geraldo_de_La_C]

I have been experiencing unusual brake failure in 2 vehicles, 2001 Ford F150, 2004 Dodge 1500. Both trucks have perfectly good brakes and operate as expected. Except in the following cases:

1 With a one day change in elevation from sea level to 8000 feet, the hydraulic system for the brakes in both trucks has intermittently failed completely, with the emergency brake being the primary way to stop the vehicle. This condition lasts for a few hours and then disappears as rapidly as it appeared.

2 The brakes function normally except during the occasional panic stop. 99% of the time the brakes function as expected however 1 % of the time even with full two foot pressure on the brakes, from about 5 to 10 mph - no lock up, no ABS vibration, simply a slow gentle stop - right into the vehicle in front of me.

Fluid level - check, Brake pads and calipers - check, rotors - check. No visible leaks.

Remember, the brakes are in working order, and they work properly 99% of the time.

I am at a loss to understand why this is happening. Any suggestions are welcome

[Skibane]

Brake pedal feels "spongy", goes all the way to the floor without much braking effect = Brake fluid is leaking past at least one cylinder, has air in it, or is boiling

Brake pedal feels firm, but requires MUCH more effort to get any braking effect = Power brake booster has an internal vacuum leak, or is no longer getting sufficient vacuum from the engine (or from the vacuum pump, if present)

[Geraldo_de_La_C]

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.

[ernie1]

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?

[jdc1]

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

[Geraldo_de_La_C]

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.

[Skibane]

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.)

[wolfe10]

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]

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

[winnietrey]

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

[gbopp]

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

[time2roll]

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.

[Gdetrailer]

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..