TSX1012m6

I replaced the front rotors/pads and did a full brake fluid flush roughly three months ago. The rear rotors are orgional (45K miles), but the pads are newer. I dont know when they were replaced (previous owner), but there is a lot of life left on the pad. After flushing the fluid, the pedal was very firm and precise.

Before replacing the front brakes/fluid change, I had weird pedal feeling that at a stop light, leaving my foot on the pedal would sometimes give out half an in a drop a bit. I assumed it was air in the lines and the problem was fixed when I flushed the system. Three months later the problem has returned. Note that this may have something to do with the heat being on in the car, and that annoying vibration that happens every so often when something kicks on, and causes the rpms to raise 100 rpms or so. Maybe this has something to do with it? Here are the symptoms:

- Occasionally sitting at a stop light, the pedal drop about half and inch after sitting for 5 seconds
- Sometimes when braking, only 1/4 of the pedal is required to stop the car/feel the brakes doing there job. Other times it requires more force (a little less that halfway of pressing the pedal)
- There sometimes is click/slight hesitation feeling when pressing the brakes. Sometimes it can be replicated completely, other times it only happens once.

As silly as it sounds, when the car is performing a normal brake using 1/4 of the pedal, it stops fantastic. When it decides to have this spongy pedal, it requires more force.

- Brake fluid level is normal. Color is perfect (very light yellow).
- No visual leaks
- No signs of stuck caliper (excessive wear, smoke, consistent noise)

The rear calipers are pretty rusted for a car that has 45K miles. My first thought is to replace the rear calipers (they cost about $110 each), rotors, and pads. After that I would perform another brake bleed job on all the calipers. Does anyone have any thoughts on what this could be? I've never worked on/diagnosed a brake master cylinder before. Other than simply "re-flush the brake system" what else could it be? This problem existed before and after the brake fluid flush.

Thank you!

BROlando

It'd be rare...but maybe your check valve for the brake booster is leaking slightly.

Its not uncommon for pedals to drop a tiny bit as engine load changes at idle from the AC turning on/off. Not sure about 1/2". Maybe....but it sounds semi significant.

The spongyness might be caused by sticking caliper parts. Check/clean/lubricate the pins and pad tips with silicone based brake grease.

benben01

I would bleed the brakes with at least 1 bottle of fluid to get all the air out. Recently installed SS lines on my C63 and it took about 2 two weeks and bleeding it 3 times to get all the sponginess out.

TSX1012m6

This is exactly what it was, I replaced the rear rotors/pads/pin boots this weekend. The rotors were original at 47K and had been through salty winters (the calipers show it too). The pads were at 90% meat left, but clearly had been through a winter. I think the rear brakes were barely working because it is a night and day difference. The slide pins desperately needed to be cleaned/lubed, one particularly had began to have rust deposits and was starting to seize. I was able to clean them back up to spec. All four pins moved very sluggishly. Everything was cleaned thoroughly and re-lubed. The pedal is back to being firm.

BROlando

Glad to hear you fixed your issue. I live in the land of crappy winter. So...I find that the brakes need some help every now and then. My slide pins are normally fine...but the pad tips and the bracket surface that mounts the pad hardware tend to swell from the rust. So I file the pad tips and coat them with a tiny bit of anti sieze or polyurea grease.

Its nice how big of a difference properly serviced brakes make in terms of pedal feel.