Brakes build pressure when hot

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IfTheSHOFits

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I'm really struggling with this one and have run out of obvious places to go. I have a LOT of information, but I'm going to summarize and save the rest for follow-up questions.

Symptom: Brakes were working fine. Stopped at a store and idled for several minutes. Upon leaving the store, the brake pedal was firmer than expected (almost the way you'd want SHO brake pedals to feel). Could tell that brakes were dragging based on increased throttle needed. Went to dinner. Car cooled. Brakes worked again; for a while. I know that the front brakes were both affected, but I didn't have an easy way to confirm the rears were (fronts were h-hot!).

As the car heat soaks, the pressure read by the ABS BRAKPRES PID slowly climbs. Have measured pressures well over 100psi. When cold, the pressure reads zero. Brake on/off (BOO) switch correctly reflects the position of the pedal.

Have tried three different ABS/HCUs in different combinations (original and two eBay). No change.

Replaced the rubber lines from frame to front calipers. No change. Didn't change the back since heat is clearly a factor and opening the hood results in a drop in pressures; so focused on areas near the heat.

Loosened the master cylinder from the brake booster to ensure rod length wasn't an issue. No change. Master Cylinder was replaced about eight months ago trying to improve pedal feel with NAPA part (not a sponsor). :). No issues in the interim. Also removed the vacuum line from the booster and plugged it to ensure it wasn't a factor; no change.

Opening up a bleeder drops pressure instantly. Pressure starts to climb a bit later.

Ran with 20A fuse to ABS module removed. No change.

Brakes have been bled many, many times during this process. even if there was air in the system, I don't expect it could reach these pressures. However, at least one of the times you'd have expected a different feel, but feel was the same after. Yes, I have a scan tool that runs the HCU bleed process.

It seems to mechanical. I'm just not sure what else it could be. Maybe the aftermarket MC is a problem where it is not fully traveling back. Maybe the flex lines between the MC and the HCU have an issue (those are not available from Ford). HCU cannot be purchased new. Used aftermarket MC as Ford didn't have them when I replaced it due to COVID.

What's left? What am I missing? Clearly something changed all of a sudden, but I'm just not sure what to try/look at next. I'm hoping the brain trust here can help me. Thank you!
 

IfTheSHOFits

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Issue was resolved. Turned out the recently replaced master cylinder had failed. I believe it was plugging the ports to the reservoir which meant as the car heat soaked and the fluid expanded, it only had one way to go. Replaced the aftermarket part with the OE version.

Picture shows the new one on top and the one being removed on the bottom.
IMG 4980
 

luigisho

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The replacement part fail throws a wrench into the works. What aftermarket brand did you use that failed?
 
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