On my 2000 F350 I rebuilt the front end and was having issues with a sticky caliper...truck would go hard left when brakes applied. It would not right itself unless you pressed the pedal hard such that the anti brake situation was activated...pedal bled down. After several months of this it did seem to get better until yesterday. I had a trailer in tow and when pressing the pedal firmly I went over a bump which caused the pedal to pulsate a little. After this now when I brake coming to a stop it does fine if I apply steady even pressure, but if I increase pressure on the pedal it steadily goes to the floor. I checked the brake fluid and all seems fine. There are no dash lights on...Brake, Anti-Lock, etc. Could this be a leaking seal on the master cylinder...or maybe a leaking seal on the other caliper?

 

Surely someone can help point me in the right direction on this...

 

I would start with the master cylinder

 

Okay, so I replaced both the master cylinder and the hydroboost as it had a small drip on the inside of the firewall. Now I cannot get the brakes to pump up any pressure. I can turn the steering wheel from lock to lock fairly easily. However, now when I turn the steering wheel the brake pedal slowly sinks towards the floor.

 

 

I put the plastic plugs in where the 2 lines...front and back...hook up, then filled the reservoir and cycled the piston cylinder several times.

 

 

You need to properly bench bleed the master first and then the entire system. Also if you let air get into the ABS module you will need a high end scan tool that is capable of a "service bleed" to get air out of the ABS, something like the Ford IDS or Forscan.

 

That's how I normally bench bleed a master cylinder but could not find my bleeder kit so it was suggested to bleed it the way I did.

I have tried to bleed the entire system starting with the back passenger, then the back driver, the front passenger, and finally the driver front. I have done this about a half dozen times and there is still no firmness in the brake pedal and the pressure at the bleeder valves is non existent...it is just a steady drip rather than spurting out.

The pedal goes to the floor easily with the engine not running. With the engine running, you can push the pedal to the floor and it will push back up a little telling me the hydroboost is working. The pedal will not build up when you pump it with the engine either on or off.

 

I did rent a vacuum power bleeder this evening and bled again at each of the four corners with no success. I am thinking tomorrow I need to power bleed the master cylinder and see what happens.

 

 

I have already bled until I am getting clean fluid and not any air...just no pressure...

 

 

Pulled the master cylinder and see the problem. There is no rod on the end of the hydroboost so they are not engaging. Anyone know if that rod can easily be installed instead of pulling and exchanging the hydroboost?

 

Luckily the parts store still had my core and I was able to pull the rod, spring, and star bushing then put it in the new hydroboost on the truck. I now have a brake pedal and good steering but am back to square 1 in that I still have a sinking pedal. There are no leaks that I can see and all four corners had good flow at the bleeder valves which to me rules out a blocked hose. I have read about the ABS unit and wondering if something in there is bypassing the fluid and what to do to correct it?