[SeanB]

UPDATE: replaced leaking MC, rebuilt calipers. Post #8.

Hi,

I'm tracking down some brake gremlins and need help. The pedal is sporadically mush and seems to have gotten progressively worse over the last year. This has me wondering if it's pad knock back, a bad master cylinder, or something else. The car is an '05 STI w/ 40K miles overall about 6K track miles. Stock Brembos, StopTech two-piece floating rotors, Motul RBF 600, Hawk DTC-70 pads, Ti backing plates, stainless lines. Original bearings seem firm. No brake ducts yet, but considering it now. Dust shields removed, lines+abs wrapped w/ reflective heat insulation.

The performance issue is most noticeable after a cool-down lap, where the pedal drops about half way and takes about 3 pumps just to start firming up (sometimes more). But more recently, it will often be mushy during the session and there isn't the usual confidence in the pedal that there used to be. The extra travel makes heel toe slightly slower when down shifting. An extra tap going into the braking zone isn't always enough and the recent lack of brake confidence is starting to show up in my lap times.

Fluid is bleed w/ a motive powerbleeder before every track event (monthly) using Subaru procedure. Front caliper fluid bleeds a dark amber, with only a few bubbles that precipitated. Strangely, I often see a few bubbles bleed out the rear calipers (both sides) which can't possibly get hot enough to boil and I'm not sure where they're coming from. A light tap w/ a soft mallet usually reveals them. Doesn't appear to be any leaks and reservoir level doesn't change measurably.


Here are a few options I've considered, although I don't have money to blindly throw into these solutions:
--Rebuild caliper seals: might help only if there is grime between the piston and bore (?)
--New front bearings $600
--New rear bearings $600


Any thoughts on how to fix this? I can't afford to lose any tenths to this again. Thanks!

[Dji]

I got the same problem at the track, I've changed my bearing all around, put a master cylinder brace. Now me too I'm looking to change the master cylinder or/and rebuilt the caliper.
If you find it, tell me.

Thanks
JF

[MGizzle]

In my opinion you can have two things going on:

1. Leaking fluid
2.

[MGizzle]

In my opinion you can have 2 things going on:

1. Leaking fluid: Your seals might be bad and you are burning off all the fluid. Stuff expands when your caliper is hot so maybe why you see a difference after cool down. Finally, you could be leaking fluid somewhere else in the system or your ABS unit might have leaky Dump cartridges that allow fluid to go into the LPA's (low pressure accumulators). this is where the fluids goes when your ABS kicks in and it unlocks your brakes.

2. You got air in the system: Fluid gone bad or just plain air in the system.

I doubt its pad knockback since you have two piece rotors. Unless your rotors are floating than yes, it could be knock back but not that bad. Three pumps is a lot and if the pedal get's firmer than something is going on.

I doubt your MC is bad too since it is just pistons pushing on fluid. If you leak past those seals the pedal shouldn't really get hard.

Keep us posted. I am curious.

[Rain]

Ive always thought it was our brake boosters that cause this spongy feeling. I could be totally wrong so just ignore me

[MGizzle]

Yeah, our boosters suck. The way it is mounted sucks even worst.

[Rain]

STi have a booster spacer, basically removes the booster from the equation, but you will REALLY have to hit the stoppers to get um working, since there is no assistance from the booster anymore.

[SeanB]

Thanks to everyone for your replies. Here's an update:

The master cylinder was the biggest part of the problem. I replaced it, and the worst of the symptoms have been eliminated.

I had been keeping an eye on the MC. The damage in this photo happened in just a couple weeks after the event I posted about. Brake fluid loves paint. Post inspection on the old MC didn't show any obvious issues, but I suspect there was contamination around the seals. Either way the leak was small; just a pin hole would compromise the entire system at these PSI levels. I regret that I didn't just replace it from the start, it's too easy and too cheap not to. Some people I've talked with replace them every 2 years.

The master cylinder was replaced with Centric part #130.47023. Expect to pay around $170. It took an usual amount of work to get the air out of the system and a small fortune in Motul.

I also rebuilt the front calipers. The seals were turning to dust, so it was a good opportunity to check on the pistons. Pistons looked good. The seal/boot kit from ProParts is your best bet. Muellerized did the work.

[Dji]

Thanks for the Pm. I cant find the seal and boot on the site of proparts. I'm buying a conversion 5x114.3 theses day to upgrade. I f after that I got the same I'll try this.
Thanks

[SeanB]

Call, ask for Jeff; he'll get you what you need.

[fweasel]

FWIW, I get knock back on my cool down lap with OEM Brembo calipers, but it takes nearly the entire course with nearly no brake input for me to get a soft pedal when I pull off the track into the pit lane.

Glad the MC solved your issue.