Uneven brake pad wear - master cylinder?

I went to swap out the Hawk Blues from my last track day - the front passenger side pads are worn almost to the backing plates - the front drivers side pads are only ~ 25% worn.

I also have recently developed a soft pedal - there are no leaks (in the calipers or the lines) - I'm thinking this problem is a bad master cylinder.

So what's the problem? I'm thinking either: 1. sticking caliper. These are rebuilt calipers, only around 5,000 miles and a year's use on them. 2. Bad master cylinder causing uneven hydraulic pressure...or... 3. Both.

I'm replacing the MC and rebuilding all calipers, just curious if anyone has come across this?

Your caliper sliders are sticking, not the piston. That's more than likely what's causing your wear.

As far as the soft pedal, that could be the MC.

Thanks Kramer, here's the pic. The backing plate also came out bent, suggesting improper installation, or...?

Stuck sliders should only wear the outer pad - the inner one is still applied by the piston. If only one side is wearing, and you have a mushy pedal - it's time to bleed the brakes.

The bent backing plate must be an installation error, which may also have contributed to the one-sided braking and even the mushy pedal - was it hooked on anything? I can't picture what it would have caught on - brakes are usually pretty idiot proof.

In reply to SkinnyG:

Thanks G. I forgot to add that the brakes were bled multiple times, by myself and a shop with no change in the mushy pedal. On track days this year, I also bled at the lunch break, with no improvement.

My mechanic made the observation that fluid might be "sneaking" past an expired seal in the master cylinder, thereby giving a soft pedal feel.

That sounds like what happened after I cooked my brakes on the track once. I'd bleed the brakes once more, even if only at one wheel, just to make sure the master cylinder seals aren't on the verge of disintegrating, although there must be other problems to cause that bent backing plate.

sliders work both ways, ie they let the caliper slide inboard to pull the outboard pad against the rotor when pressure is applied to the piston, and they let the caliper slide outboard when pressure is released to avoid dragging the outboard pad against the rotor in off-brake operation.

i've seen calipers (specifically the rear calipers on miata and probe gt) with sliders so boogered up that the calipers couldn't slide inboard, which means they couldn't pull the outboard pad against the rotor. inboard pad worn to the steel backing plate, outboard pad barely worn.

GameboyRMH wrote: ...there must be other problems to cause that bent backing plate.

Yeah - one really stuck slider might do that to the pad as it allows the caliper to twist.

That would also explain a mushy pedal since the piston is pushing against a flexing bunch of metal instead of squashing the pad material into the rotor.

clean up the sliders, replace the pad, bleed the brakes.. and then test. Once you have gotten the cheap stuff out of the way, you can worry about the MC

Price out new slide pins as they are usually only a few bucks; it can make cleaning them seem pointless when a new set can be had so cheap. Also, hoses can cause symptoms like this if they are still rubber. Internal cracking can create a valve like scenario applying the brakes but not releasing them. Personally I'd suspect a bad install and frozen slides but there are a few other options.

Johnboyjjb wrote: Price out new slide pins as they are usually only a few bucks; it can make cleaning them seem pointless when a new set can be had so cheap. Also, hoses can cause symptoms like this if they are still rubber. Internal cracking can create a valve like scenario applying the brakes but not releasing them. Personally I'd suspect a bad install and frozen slides but there are a few other options.

I've also seen a bubble in a hose cause a very hard to find spongy pedal. Look for balloons sticking out of braided steel lines or old factory rubber - start where they might be rubbing on something.

Replace your flex lines with new units, you may have one collapsing internally contributing to your spongy pedal (on top of the stuck/sticking calipers)

Good luck!

You could also have a leak at the caliper. I've got one on my Miata. The fluid is leaking past the piston on the drivers side. Spongy brakes and uneven wear ensues.