This car is perplexing. Customer complaint of intermittent brake pedal drop. Mainly while sitting at a stop. Perform full brake inspection. Line clamp test. Could not duplicate. Fascar strip showed dark purple, performed brake fluid exchange, we thought just MAYBE it was brake fluid boiling on San Francisco downhill stops. Fast forward a few months and it's back. Same symptoms. In fact it acted up a few days after the last visit. Fleet vehicle, they just have not been using it. Still cannot duplicate symptoms. Made customer aware that this seems like a classic master cylinder issue. Checked Identifix, AND google, saw LOTS of master/booster issues. Replaced both AND check valve. Test drove for a few days. Seems fine. Customer comes to pick it up. Pedal to floor before even moving the car! Okay, get another MC from Ford (oem) and this one won't even bleed out one port. Brand new outta the box defect. Got ANOTHER master. Rayloc. Seems fine. Bleeds easily. Pedal high and hard. EXCEPT sometimes. This happens without even having moved the car. So even though bearings are good and no rotor runout to speak of, I have eliminated caliper piston pushback as a cause. Caliper slides are all free and move okay.

Am I correct that the ONLY other place to look is the ABS modulator? There are NO codes, ABS functions normally on hard stop. Expensive parts to replace without having a valid way to prove short of substituting "known good unit"

Gary Siegel
Owner
San Francisco Auto Repair Center Inc
San Francisco, California, USA