Brake pedal goes to the floor while applying brakes?

MBUCKHAN

MEMBER
2003 FORD F-150
145,000 MILES

Starting out I have a brake pedal that stops the truck, but when I sit at a light the brake pedal does all the way to the floor and the truck inches forward. The master cylinder has been replaced to try and correct the issue, all four wheel seals were checked and no leaks found, all brake lines were bled and have no leaks and the brake fluid stays at the proper level, so the system does not have a leak. Could the ABS box be the issue and what in the ABS box would cause this problem?

6 Replies

HMAC300

MECHANIC
48,601 POSTS

It can take up to 3 qts to bleed brakes on an abs system was the master bled prior to installation? if not that is your problem you'll need to bleed again preferably with a power bleeder or vacuum. here is a test to do on abs unit.

It sounds like the master is bad these guides can help us

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

and

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

Probably abs hydraulic unit. If it is 2wal you should find a rubber cap on one end of the release valve. Take the cap off and stick a straightened paper clip in the hole. Have someone push on the brakes, if it pushes your paper clip then the 2wal valve is bad. Good luck

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Wednesday, March 10th, 2021 AT 11:00 AM

CJ MEDEVAC

MECHANIC
11,004 POSTS

The master cylinder could have a so called "internal leak", where as the plunger inside is not sealing in the bore 100% fluid slowly leaks by this can happen even with a brand new unit.

Rebuilding or replacing the master cylinder is the fix

The Medic

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Wednesday, March 10th, 2021 AT 11:00 AM

CHRISTOPHER NULL

MEMBER
11 POSTS

So I have been reading lots and lots of posts on this issue. How can 4 or 5 M/C be bad in a row? That just seems unlikely. My best guess so far is ABS unit or pump.

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Wednesday, March 10th, 2021 AT 11:00 AM

KEN L

MASTER CERTIFIED MECHANIC
47,513 POSTS

Yes this problem is common the ABS valves need to opened using a scanner and then bleed the system.

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Wednesday, March 10th, 2021 AT 11:00 AM

CHRISTOPHER NULL

MEMBER
11 POSTS

My friend that works at a garage found this on the Mitchell's manuals site for a 2000 F150

If equipped with the four wheel� anti-lock� brake system and the Hydraulic Control Unit (HCU) has been replaced or is suspected of containing trapped air; bleed the brake system using the procedure below. Conventional brake system bleeding cannot remove the air trapped in the lower portion of the HCU. If a spongy� or sagging brake� pedal� is present and air in the hydraulic control unit is suspected, use the following procedure:

1. Bleed the brake system as outlined in the conventional bleeding procedure.
2. Connect a New Generation Star (NGS) tester or equivalent scan tool, to the serial data link connector below the instrument panel as though retrieving codes.
3. Make sure the� ignition� switch� is in the RUN position.
4. Follow the instructions on the NGS screen. Verify correct vehicle and model year go to the "Diagnostic Data Link' menu item, choose� ABS� Module, choose "Function Tests", and choose "Service Bleed."

5. Bleed the right-front wheel as follows:
---A. Open the caliper bleed screw and pump the� brake� pedal� for 3 seconds. Repeat the procedure again.
---B. When the fluid runs clear, begin the program and continue to pump the� brake� pedal.
---C. Continue bleeding for approximately 1 to 2 minutes after the program ends and then tighten the bleed screw.
6. Repeat the bleeding procedure to the left-front, left-rear and finally the right-rear wheel.
7. Remove the pressure bleeding device and adjust the� brake� fluid� level.
8. Road test the vehicle and check for proper brake system operation.

This is what you have to do. Period. If you want the brakes to bleed off properly