I just replaced the master cylinder and bled the lines. With the engine off the pedal is hard and doesn't fade at all but when the engine is started the pedal starts firm but fades all the way to the bottom. To be fair, I'm pushing pretty hard to test it, but it shouldn't fade. Also, now the brake light and the ABS light are on.

I've bled 4 quarts of fluid through the system and I'm not getting any bubbles at all, there aren't any leaks. The problem doesn't seem to change which indicates to me that there's still trapped air somewhere.

What am I missing?

 

other than the lights its pretty normal for the pedal to drop slowly to the floor. Well normal for these trucks, Ford has come out with a zero lose brake boost to "fix" this. all it is is that when you push the brake down you deplete the vaccum assit some and the vaccum pump rebuilds the vaccum so the booster keeps dropping. The lights could be an ABS issue as some times the ABS issues will also bring the Red Brake light on.

Did you bleed the RABS valve on the frame rail under the drivers floor board? there is one there.

 

TJ,

Thanks for the quick reply. I recently learned about the RABS valve and will bleed it along with the rest of the system again tomorrow.

I had to replace the master cylinder once before and frankly the brakes never did feel right. The the brake and ABS light came on then too and stayed on for several months. Then one day I started the engine with my foot pretty hard on the brake pedal and I felt some funny kind of feedback (I can't remember exactly what happened any more) and the lights went out and stayed out until now. The lights always come on while waiting for the glow plugs so I know they are working. I tried this trick several times today but the lights are still on.

Jeff

 

Did you bench bleed the master cyl , before you installed it?.....

 

I did fill the reservoir on the bench and exercise the plunger while capping outputs. I may not have followed strict bench bleeding procedure but I did prime the system prior to installation.

 

Hmmmm........

 

I know that when I replaced the master cylinder on my Silverado I had the same problem. The answer is usually that the ABS pump has air in it. Shops use a tool that plugs into the truck and cycles the ABS pump to push the air out, but you can also accomplish the same thing by braking hard enough to trigger the ABS. That pushes the air out of the pump and you can then bleed the brakes normally.

It sounds as if the RABS valve accomplishes the same thing.

Hope this helps, and sorry for using the word Silverado in a Ford forum