I am assisting a friend that is dealing with some hardship. In this process I am trying to get her truck drivable, It is a 2003 F-150 2WD 4.6 V8. First reported symptom was very soft brake pedal. I first checked the pads and they were in good shape (about halfway through the life). The truck has 240K miles on it. I had to change the front calipers because the bleed screws were frozen. I went ahead and changed the front pads with the calipers. Not much help. After bleeding all four brakes I still didn't think that the pedal was firm enough, so I went ahead and changed the master cylinder and re-bled the brakes. After bleeding I get a good solid pedal with the engine off, but as soon as I start the engine the pedal goes to the floor. It will pump up a little, but not enough to make me feel comfortable. When I turn the engine off I have to press the pedal a couple of times, but it comes back to solid. I went through the tech forum and read that sometimes the ABS pump can cause this symptom. I went through the process of removing the plugs and made measuring rods and checked the piston movement. I got 3/32 on one and 1/64 on the other. Any help or suggestions would be greatly appreciated. I've been working on cars and trucks for over 50 years and can't figure this one out. Thank you for reading!

 

Did you check the gap of the brake booster push rod and the new master cyl.? I'm not sure of the spec. on a 2000 but on my 97 Ford says about 1/2mm. I only have 2w abs so no pump I read that some people had to use the abs scan tool to bleed it well.
Airborne

 

Thank you for the insight. I will check this. Also, I want to THANK YOU for your service. I lost my father in the Air Force and I along with several members of my family have been in various branches of the military.

 

If you do not have the proper tool to measure the gap some have good luck with the go-no go method. Adjust the rod in to a no brake drag point then out until the brakes just starts to drag then back it back in 1/8 to 1/16 turn. It does not take much to get a 1/2mm gap. To much gap + low pedal no gap + brake drag. I'm thinking you still have air in the ABS pump. Some have a hard time getting it all out and some get it the first time
Airborne ALL AMERICAN

 

What Airborne say but another thing what happened to me is I changed master cylinder at Acura legend like 15 years go and had to bring 5 different from youpull till I figured out that master cylinder MUST be bleed. Buy master cylinder bleed kit and bleed it per directions after it is installed on truck. Need not bench bleed it. Definitely a must, if you didn't bleed MC it is it. MC must be bled at his own with special kit for it.

 

You can get a short 12" brake line with fittings on both ends at NAPA. cut that line in half now you have two 6" lines to screw into your master. bend them so they flow back into the reservoir and pump away. No need to keep refilling.