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The knee jerk response is "your master cylinder is bad". But wait...



I noticed after replacing the driver side bracket and caliper, that when the engine is running and heavy pressure (but not standing on the brake pedal heavy) the brake pedal very slowly over 30-60 seconds sinks to the floor. There is no brake fluid leak. The pedal feels firm when the engine is off, and drives normally with normal feel for brakes, and I can sit at lights for many seconds without any noticeable sinking of the brake pedal-- i.e., under regular use brake pedal pressure. Vehicle is 2001 F350 7.3L, hydroboost replaced about 6 months ago.



I notice the same thing with a conveniently available F150 (2005, 5.4L engine). Driving is normal, but engine running with heavy brake pressure the brake pedal slowly sinks to the floor over 30-60S.



Now, is this brake pedal sinking normal? Is there really a slow (abnormal) leak past the master cylinder seals? Is this a side effect of the ABS hydraulics? I don't want to replace the master cylinder and maybe have to bleed the RABS unless there really is something wrong.



Thanks.
 

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I'm guessing that you got some air in the system that needs to be bled.
 

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Could be air. I have had it happen on a 2014 F150 as well, however there was also a recall for the master cylinders on the 2014 F150, so who knows. One way to know if there is air is let it sit a week, then press on the pedal without starting the truck, if there is air the pedal will sink more than usual and pump up firm with a few strokes. I just did the brakes all around on a 95 Grand Marquis and it did just this, turned out one line had a leak under pressure, replaced the copper washers, and rock solid pedal.
 
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