The brake pedal goes to the floor?

Tiny
HBB
  • MEMBER
  • 1997 FORD E-SERIES VAN
  • 5.4L
  • V8
  • 2WD
  • 118,000 MILES
We've replaced pads, master cylinder, a caliper, a rear wheel cylinder, booster, and bled the heck out of the system

However, the pedal holds tight until you turn the engine on, then the pedal goes to the floor. Does stop the vehicle, but goes to the floor.
Wednesday, March 23rd, 2016 AT 9:42 AM

3 Replies

Tiny
HMAC300
  • MECHANIC
  • 48,600 POSTS
You probably have air in abs unit, try a power bleeder and it can take up to 3 qts if it still doesn't work you need to have apro use his scanner to bleed it to open valves on abs while bleeding.
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Wednesday, March 23rd, 2016 AT 12:26 PM
Tiny
CARFIXES
  • MEMBER
  • 4 POSTS
Hi HMAC300. You said "have apro use his scanner to bleed it to open valves on abs while bleeding."
1) Can you please explain whether the scanner will automatically do the opening of the 4WABS OR will the scanner instruct the person doing the bleeding to manually open the 4WABS?
2) Can you please attach a picture of what the 4WABS looks like and where it is located on this year and model.
Thanks
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Saturday, June 24th, 2023 AT 4:28 PM
Tiny
CARADIODOC
  • MECHANIC
  • 33,783 POSTS
I can help with that question. If the reservoir is allowed to run empty during bleeding or when a hose or line springs a leak, air goes down to the wheels, and some of it gets trapped in the high spots in some of the chambers in the ABS hydraulic controller. It pools there unless at least two valves are opened to let that air continue on down to the wheels.

The only way you can make those valves open is to drive the vehicle and put it into a skid. The ABS Computer will cycle the valves, but since no bleeder screw is open, the fluid and the air in it have nowhere to go. You would need to run alongside the van, and reach behind each wheel to open the bleeder screws while the van is traveling at least 9 mph, and the wheels are skidding. I'm not recommending you try that.

Instead, the scanner has a program that commands the ABS Computer to open some valves while the bleeder screws are open. The scanner tells you which bleeder to open, typically the right front, and to press the brake pedal while you press the next command on the scanner. It will tell you to close that bleeder screw, open the second one, usually the right rear, then do the same thing. Once the scanner is connected, if you're doing this with a helper, the procedure takes the better part of half a minute.

The idea is the air has made it out of and past the chambers, then regular bleeding will get it all the way to the bleeder screws. Most of the time the air makes it all the way out of the bleeder screws without the need for additional bleeding.

There are a number of tricks to prevent this from happening. When you're replacing a caliper or wheel cylinder, use a stick from the front of the seat to hold the brake pedal down about an inch. Gravity won't be strong enough to pull brake fluid out of the reservoir and past the rubber lip seals in the master cylinder.

When replacing a master cylinder, it is not necessary to bleed at the wheels. Doing so just runs air back down to the hydraulic controller and that misery again. After bench-bleeding the new master cylinder, install it, but leave one of the soft metal line nuts loose. Have a helper push the brake pedal slowly halfway to the floor. It should take about 15 seconds. You'll see air bubbles coming out around the line nut. Snug that nut, then have your helper release the pedal quickly. Do that a second time and even a third time until you see only brake fluid coming out.

By pushing the pedal slowly, brake fluid runs down the lines, but any air bubbles have time to float back up. When the pedal is released quickly, any sticking air bubbles will wash back up into the reservoir with the returning brake fluid.

Do that for the second line. Many Fords use four separate lines at the master cylinder. Bleed all four the same way. Even if you don't bother with that bleeding step, air always floats up, not down toward the wheels. As you're sitting at a red light with the engine vibrating, any air bubbles will eventually float up, then when the brake pedal is released, they go into the reservoir and are bled out. Those few bubbles aren't nearly enough to cause a noticeably low brake pedal.

Be aware too a real lot of master cylinders are damaged by improper bleeding techniques. The brake pedal should never be pushed more than halfway to the floor. Crud and corrosion build up in the lower halves of the bores where the pistons don't normally travel. By pushing the brake pedal all the way to the floor, the rubber lip seals are run over that crud which can rip them. That results in a slowly sinking brake pedal, and that often takes up to three days to show up. That warning doesn't apply to new or rebuilt master cylinders less than about a year old. That corrosion hasn't had time to develop yet, but it's still a good practice to get into.
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Saturday, June 24th, 2023 AT 7:31 PM

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