96 Corolla Spontaneous Violent Pulsating Brakes

[christensent]

I don't use my car much, and park it on the street. I last drove it 3 weeks ago, and it did great on a 6 hour trip. It is worth noting that until this year, the car had lived in Washington, and this 6 hour trip was the first welt salted road experience the car has ever had.

Today, I go to drive it, and the first time I hit the brakes, I'm greeted by the entire car shaking and the brake pedal pulsating. My first thought was "wow, are the roads really that icy? that's a lot of ABS...". I then realized this was absurd, the roads are clearly dry and this doesn't quite feel like ABS (although it's a very similar feeling at high speeds). My next thought was that I must have ice in the pads, it'll melt off soon. So I drove 20 minutes, and it didn't improve. I turned around, and drove home.

When I first moved the car, I had to give it gas to get it to go and heard a snap. I assume this was it being stuck in ice as I hadn't driven for a while and it was very cold. It could have been something breaking, though.

I have since taken off all the wheels, visually inspected the drum brakes for any damage (pads look fine). I looked at the disc brakes and they looked fine. One of the caliper slide pins was completely seized, so I was hoping that'd fix it, but it had no effect at all.

The brake fluid level is full. I seriously doubt this is warped discs, because it drove perfect last time I used it and it was spontaneously so bad today.

So, what do I do now? I don't know a whole lot about cars but I'm pretty good with fixing stuff if I know what to do, have shop access, and would rather not pay someone to fix it. Any ideas?

[Brian R.]

I would try flushing the braking system with new brake fluid and checking the pistons for rust or a frozen piston.

[christensent]

I flushed and replaced the fluid today. The pistons appeared to be in great condition. One of the bleed valves sheared and could not be removed, so I also replaced one of the calipers (unfortunately had to do both at the same time, so no way to know what it was).

The problem is 90% better, but it's still 10% there when moving over 30MPH. It is no longer observable for less than that speed where previously it was. Dial indicator shows disc flatness to be a little better than 0.001".

Do you think it'd be worth the money to replace the other caliper to see if that finishes it off? Seems weird that I'd have a simultaneous caliper failure. The problem was after it was sitting in quite cold weather (first sustained below-freezing temperatures of the year) for 3 weeks, and based on the reports of the previous owner (my parents), the brake fluid may not have been changed for 10+ years and 150k miles. Could freezing water in the crappy old brake fluid actually destroy the calipers?

[christensent]

Finally got this solved. I was hesitant to replace rotors since they measured flat from the outside. But I finally gave in and figured before I take it to a shop I might as well replace pads and rotors. Somehow I didn't notice this before, but the inside of one side has the pad deeply grooved on the inside and outside, and the rotors are grown out in those regions. The rest of the rotors have the outer edge crumbling away, but on this one it's expanded out and was carving the pad.

Replaced everything and all is good now. But what's up with this? How did this happen? Image is pad sitting on top of rotor.