I had a couple of times experience a weird thing happening with my brake pedal, just last week me and a friend did the brakes on my '97 F-150, we took the front rotors to be machined and put new pads on the front, my friend is a qualified mechanic so we supposedly did everything correctly, we even bleed the brake lines cause I had a bad caliper which I had to replace.

The problem just happened yesterday, in the morning when I first was warming up the truck, took it outside the house and I notice that the brake pedal was a little higher than normal, I had to press it less to have full brakes on, so I drove the car off the driveway, I have to do a couple of full brakes and I wasn't 100% sure I was having to press the pedal less, so I went about 50 meters to the corner on my street to the first stop sign, and when I applied the brakes and was on a full stop, the pedal started to go down little by little, I would even hear a little air like sound, the brake pedal would go like 1/8 of an inch down, and stop for a split second, then go another 1/8 of an inch and stop again, and so on, the truck would not move, would still be on a full stop although I was on a incline and I think that to go off that incline I have to press the gas, so I let go the brake pedal go and pressed it back again, and the pedal went a little lower than normal, I let go the brake pedal again and pressed it in, and the pedal went back to normal and did not failed again during my drive to work, I am concerned cuz I think this happened before but I did not let it do it cause I let go the brake pedal before it went down every split second.

I think I may have one of two problems, either I have a bad master cylinder, or air on the brake lines, do you guys think I am on the right track on this? I want to bleed the lines again tomorrow, I have a can of brake fluid ready to go, should this fix the problem?

Hopefully I described the problem successfully, English isn't my first language.

 

Keep bleeding them.
What bleeding method did you use?
I had the same problem recently. I replaced the Master, then the brake booster, and kept bleeding them.

Another thing to check before you decide to change anything else, is the check valve on the brake booster. When you turn the truck off, and let it sit for a few minutes, there should be enough vacuum still in the booster for another assisted brake depression. If not , I would crank the truck for a minute, and then turn it off, let it sit for say five minutes. Then wiggle the check valve to see it there is a vacuum sucking noise between it and the booster. If no vacuum is heard replace the check valve.

If there is a suction there post up, and someone will tell what you need to check next.

Edit: A new check valve also comes with a new booster, and I'm not sure exactly what fixed mine. My youngest Son had been driving it, and it may have been acting like yours before the wheel cylinder change, so I'm not sure which fix actually fixed it. I do know the problem didn't completely go away, until we re bled several times. But I'm almost positive it was just the check valve and the wheel cylinder that started the problem.

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To be honest, we used the only bleeding method I know, my friend opened the valve on the caliper, let some fluid come out, I would be inside pumping 3-4 times, he would tell me to stop, and he would do something, I was inside the truck I don't know exactly what he did, but I had to repeat this like 5 times on each caliper.

I didn't had enough time to do the brake lines again, I put new anti-freeze and a new thermostat on my truck and didn't left enough time or beer to do the brakes again, the brakes have not failed, but my friend believes I have some air on the lines, we should resume next Saturday, hopefully somebody has any more ideas. Thanks Tumba