Brake Failure, Brake Failure Options

[Rod911964]

914 Brake failure!
Took my 914 1.8 on a 2300 mile rally this past autumn. 914 did great except the brakes went spongy. So I bled the brakes thinking the was air in the lines. Actually bled them 3x.
On the way home I had to pump the brakes to get any braking action, ugh!
Got home and put the 914 on the lift and started the diagnostic process. Upon the 1st round of inspections… no leaks, reservoir maintains fluid level, no indication of caliper hang up.
Took the master cylinder off and disassembled, scoped the bores and found annular rust in the bores. So I figure I’m getting fluid bypass to not hold fluid pressure. Probably the root cause.
Then I take the pads out to measure both the rotors and pads to spec. All looks good except for the front left pads. Uneven/slanted wear! (See pics). If I hold the friction surfaces parallel it would mean pistons are cocked. Doesn’t make sense. Or the rotor surface is not parallel.
So what causes this situation?
Thanks in advance.
Rob

[technicalninja]

I see wear like that all of the time!
Jammed slider pin on a "floating" caliper like found on 95% of everything modern.
The 914 uses a fixed caliper so it rules a jammed pin out...

The lower pad appears more tapered than the upper pad to me.

Possible causes IMO:
Bent spindle/caliper mount pads.

Pistons not clocked properly.

Too much rotor run out. Fixed calipers are far more sensitive regarding rotor runout.
This should be checked ON THE CAR because the wheel bearings and the spindle can affect the run out.

Check piston for strange machining. The lower pad is worn like the piston is hitting it at a 10-degree angle.

Piss poor pad material. You could just have a fluke and the pad material on one pad was defective.

Loose mountings. Figured you would have said something if the one caliper was loose.

If it was me, I'd rebuild the calipers (fronts are easy), change pads, turn/replace rotors
Service wheel bearings, and flush/bleed brakes. I'd look at the rest of the brake hydraulics with a microscope.

I'd watch that like a hawk and if it did that same thing again, I'd be stumped...

Very strange wear for a fixed dual opposing piston caliper.

If it did it again, I'd start with a different strut housing/spindle.
It looks like the centerline of the caliper is offset from the centerline of the rotor. (but more on one side) WTF???
If it's bent badly, I would think I would be able to see it when I tried to line up the parting line between the two caliper halves with the rotor centerline.

Please post what you come up with