Brake pedal goes to floor and rear anti-lock light on

Hi and thanks for using 2CarPros.com. If there is no brake fluid in the master cylinder, you have a leak. The ABS light will come on under these conditions. Now, you need to locate the leak. Primarily, the brake lines are made of steel. There are rubber hoses at the front brake calipers, too. Most times, the steel brake lines will rust and burst. Thus, you lose the brake fluid and braking ability. It is also possible for the rubber brake lines to rupture and leak. So, you will need to check all of them. If you find no leaks, there is a chance the master cylinder is leaking from the rear and filling the brake booster with the fluid.

Here is general information related to the brake going to the floor:

https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

Here are directions specific to your vehicle for checking for leaks:
INITIAL INSPECTION AND DIAGNOSTIC OVERVIEW
INSPECTION

Inspect for Leakage

NOTE: The master cylinder should be cleaned thoroughly prior to inspection (several days preferably). Spilled brake fluid can be easily confused with leakage.

Piston Seals
- Leakage around the piston seals will seep out from the end of the cylinder and leak down the outside of the brake booster.
- Inspect for signs of seepage or bubbled paint on the brake booster.
- If any leakage is detected the master cylinder should be replaced or overhauled
Brake Line Fittings
- While someone is applying pressure to the brake pedal, inspect the brake line fittings on the master cylinder for leakage.
- If the fittings are leaking they should be disassembled and inspected.
Reservoir to Cylinder Gasket
- Inspect the gasket for signs of leakage or hardening and cracking.
- Overhaul or replace the master cylinder if any leakage is indicated.
Inspect for Oil Contamination.
- Inspect the rubber gasket on the inside of the master cylinder lid. If the gasket is swollen or bloated, the brake fluid is contaminated with oil.
- Siphon or scoop a small amount of brake fluid out of the master cylinder and place into a styrofoam cup filled 2/3 of the way with water. Brake fluid will mix with the water while oil contaminants will float on the surface and dissolve the styrofoam.
NOTE: If any oil contamination is present ALL brake components containing rubber seals will need to be replaced or overhauled. This includes the master cylinder, both front brake calipers, both rear wheel-cylinders, and all flexible brake hoses.

Verify Hold-Down Bolts Are Tight
- While someone is applying and releasing pressure to the brake pedal, verify the the master cylinder is tightly secured to the brake booster.
- Retighten hold-down bolts if necessary.
Functional Checks

WARNING: Always bleed the entire brake system and verify the rear brakes are properly adjusted prior to diagnosing a master cylinder as defective. For additional information see Master-Cylinder / Brake Pedal Relationship.

Check Related Systems
- Check for proper brake booster operation
With the engine "OFF", depress and release the brake pedal 4-5 times (this bleeds off the vacuum reserve in the booster).
Depress the brake pedal firmly and start the engine. If the pedal goes down slightly the brake booster is assisting in brake operation.
If the pedal does not go down slightly the brake booster is not assisting in brake operation and should be diagnosed for problems.

- Check and adjust rear brakes.
- Check all brake lines and brake assemblies for leakage.
Repair all deficiencies with related systems prior to diagnosing the master cylinder as defective.

Check Brake Pedal Travel (Non-Antilock Brake Systems Only)
- Fully depress the brake pedal and check for the following:
- The pedal should have a 75% reserve.
Pedal Reserve Checks

- The pedal should be firm and not leak down.
Replace the master cylinder if all of the following conditions exist:
- The brake pedal is not firm with a 75% reserve and -
- There were no problems with any of the related systems and -
- The entire brake system has been bled and -
- The rear brakes are properly adjusted.
'
The easiest way to find the leak is by simply refilling the master cylinder and then pumping the brakes. If there is a leak, it will push the fluid out at that point.

Let me know if you have other questions.

Take care,
Joe

Has to be a leak if the reservoir is empty. When you cannot find an external leak, a good suspect is the rear seal in the master cylinder is letting brake fluid get sucked into the power booster, then it gets burned in the engine. To identify that, remove the master cylinder's mounting nuts, pull it forward, then check for wetness around the push rod area.

If you need to replace the master cylinder, given the age of the truck, there is a good chance the bleeder screws on the calipers and wheel cylinders will be rusted tight and may break off. Here is a trick to avoid having to bleed at the wheels.

When you replace the master cylinder with two steel lines, loosen the line nuts a little, remove the mounting bolts to the power booster, pull the master cylinder forward, then use it as a handle to bend the steel lines up a little. That will keep the fluid from running out of the lines.

Remove the two lines all the way, then remove the master cylinder. Brake fluid eats paint, so be careful to not allow any to drip onto the truck.

Screw the two lines into the new master cylinder that has been bench-bled, then use it to bend those lines back down to their normal shape. Bolt it to the booster, then snug one of the line nuts. Have a helper slowly push the brake pedal half way to the floor. It should take about fifteen seconds to do that. You will see bubbles coming out by that nut. Snug the nut, then holler to the helper to quickly release the pedal.

Do that a second time, and perhaps a third time, until you see only clear fluid with no bubbles coming out, then do that for the other line. By pushing slowly, fluid will get pushed down the lines, and air will float back up. By releasing the pedal quickly, the fluid rushing back will wash the air back up into the reservoir with it. This can even work when working on the truck by yourself, just keep the line nuts tight.

This wondrous trick might not work on Fords that have four lines at the master cylinder.

It is good practice to never push the pedal over half way to the floor, although it will not matter with a rebuilt master cylinder. Once they get to be about a year old, crud and corrosion build up in the lower halves of the bores where the pistons do not normally travel. Pushing the pedal to the floor, like most do-it-yourselfers do, runs the rubber lips seals over that crud and can rip them. That results in a slowly-sinking brake pedal, and that often does not show up until two or three days later.

During this repair when there was no other leak, there is no way any air got into the steel lines. If there is still a few tiny bubbles stuck near the master cylinder's ports, it is going to wash into the reservoir eventually with repeated pedal applications. You will never notice a low pedal due to that small amount of air.

Until I get back here to see how you are doing, here is a couple of articles you might want to look at:

https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor
https://www.2carpros.com/articles/how-to-replace-a-brake-master-cylinder