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

Click to view attachment

I don’t know the problem but the rotors have parallel sides.

Are the 'piston retaining plates', those 20 degree angle things holding the pistons
in the proper orientation per the manual?

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

Pistons won't tip over with pads that thick or even ones more worn, all i can think of is bad wheel bearing but that would have been very noticeable ??? not seen that before somewhat odd ,cheers

I'm with porschetub, wheel bearing bad or not tight.

update to my original question/concern.
I talked to a rebuild tech at PMB Performance about my uneven brake pad wear (see pics). PMB said this is caused by the piston not being clocked properly. So I took the front calipers off and yes the piston had rotated over the anti squeal alignment piece. PMB said the see this all the time. Blew out the pistons, cleaned the pistons and bores, checked the o-rings reassembled. installed a new 19mm MC, bleed the MC and caliper/lines. Oh yea, new pads for the fronts.

went for a test drive to bed the pads and test the brakes. The brakes work great, plenty of threshold braking cycles and heat soaks. I still have some "brake job anxiety". What is the typical brake pedal movement before pad engagement? My pedal moves about 1/3 the range before braking action. Maybe I need to bleed the MC and lines again. I'm hopeing for a rock hard pedal at the top of pedal range (like my RSA).
Thoughts, suggestions, experiences are appreciated.
Rob

Yup, looks like simple piston clocking is needed. You’ll never get a rock hard pedal on a 914 with stock mc and calipers. Go to a 19mm mc and you’ll have a harder pedal. Can’t hurt to bleed one more time, but sounds like you’ve got a stock 914 brake feel…

A couple of things to try to get a hard pedal near the top of the movement range:

- Set the venting clearance in the rear brakes. Use half the spec from the manual on each side.
- Fabricate a brace that keeps the whole master cylinder from moving when you press on the pedal. The floorboard where the MC mounts can flex under hard pressure, and that feels like a soft pedal and moves the actuation point further down.

If you're really ambitious, you can try reducing the rear venting clearance a little bit, or reducing the free play on the pedal's pushrod, but those can cause problems with (e.g.) the brakes not releasing fully when warmed up or similar. I don't really recommend that, though.

--DD

A low pedal is typical after a caliper rebuild. The new seals often grip the pistons a little too tightly and won't allow self adjustment until a few hundred miles of operation.

I'm also thinking new pad compression/heat cycled break in. Old pads have been compressed under heat and pressure for thousands of miles and the surfaces are very hard. New pads aren't broken in yet so they don't have the compression resistance.