Inconsistent brake pedal

[MauiRoads]

In previous posts, mostly about FORScan individual component bleeding (https://www.fiestastforum.com/threads/how-to-bleed-brakes-with-forscan.29950/page-3) I thought a full FORScan bleed had solved my inconsistent brake pedal. Turns out, it reduced the inconsistency, but didn't eliminate it.

Sometimes, hot or cold, the pedal has little initial travel before reaching an acceptable firm feel. Sometimes, even seconds later, hot or cold, the pedal has more initial travel and and an unacceptable soft feel. I've tried all of the FORScan bleed functions and even got FORScan to send me the "European Version" with individual component bleeds (MC, ABS, individual calipers) without improvement. Since then, I installed StopTech slotted rotors and Porterfield R4-S pads, and cleaned and regreased caliper slide pins and pad sliders. While the R4-S pads greatly reduce the factory pads' initial grabbiness and are more linear, the inconsistent pedal remains.

I haven't tried the cruise-control switch, though I've never understood how that can affect brake performance...doesn't it just switch off the cruise control when you press the brake pedal?

Does this sound like a booster problem? I have no experience diagnosing those.

Any help would be appreciated.

 

[MauiRoads]

Has anyone else experienced this, or a similar brake problem?

 

[Intuit]

If the brake booster were leaking vacuum, the pedal would initially be hard and get softer. The more vacuum the engine pulls from the brake booster, the more assist it can provide. (to a point)

The brake booster does have a one-way valve that prevents it from leaking vacuum back into the intake. If this has a problem, you might theoretically end up with some of the symptoms you describe. But this problem would be dead obvious as you would have no brake assist before starting your car at the beginning of the day... your pedal would be hard as a rock. Vacuum is highest when your engine is running with the throttle plate closed.

Put all four wheels up in the air. Run the engine for a short while at idle. Go to each wheel and give it a roll. Apply and release the brake. Then repeat the prior test. They should all be similar resistance. Take the vehicle for a drive and perform some longer, harder stops to heat the brakes up nicely. Repeat the prior tests before they have much of a chance to cool. It's common to have caliper slide mount(s) that only lock up when hot. https://www.fiestastforum.com/threads/caliper-slide-mounts-the-permanent-fix.29023/

 

[MauiRoads]

Thanks Intuit. I'll give those suggestions a try and report back. More likely its something other than the caliper slide mounts: I just cleaned and lubed those, and the inconsistent pedal happens when the brakes are cold or hot. But I will run the hot test you suggested. I'll also start paying attention to whether the symptoms have any relation to engine rpm.