Originally Posted by Darth Evader
I was wondering if anyone might know what would cause my brake pedal to become softer and travel further as RPM increases above idle. It only takes increasing idle RPM by 50-100 for this to become noticeable. For example, even the extra RPM from having AC is creating a mushy pedal.
Shouldn't the pedal become more firm since the engine's vacuum is decreasing?
I suspect that you have a small vacuum leak somewhere in the power assist line (probably closer to the PB booster) or possibly in the booster itself or its diaphragm.
You can check this yourself if you have access to a vacuum pump and a gauge (you'd be watching for the vacuum level on the gauge to steadily drop after briskly pumping it up).
I think if you pumped the brakes a few times on any car with vacuum assisted brakes without starting the engine (to ensure that no PB vacuum was present), then held your foot on the brake pedal as you started the engine that you would feel pretty much the same softening and additional pedal travel.
Ditto on flushing the fluid. A conventional bleed with the pedal or a simple gravity bleed will clear everything in the system except for parts of the fluid circuitry within the ABS unit. For fluid, look for higher wet boiling points first, then high dry boiling points. I'd preferably use a quality DOT 4 rated fluid, or maybe Ford's own OE fluid. Stay away from any minimum-spec DOT 3 fluids that you might find for cheap at the parts stores. FWIW, I'm running ATE Amber, sourced through Sam Strano
. . . . (over a long enough time the blue stuff can stain plastic making the fluid level in the reservoir harder to see, not to mention that it is technically an illegal fluid for use in Florida).
'08 GT coupe, 5M, wheels, tires, pads, fluid, a few suspension mods . . . finally lowered (all of half an inch)
'10 Legacy 2.5GT, 6M (hers) . . . '01 Maxima 20AE, 5M (spare, winter driver)