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Old 12-04-2018, 12:35 AM   #1
corona672
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Default Brake Master Cylinder Failure

I had a brake failure a few weeks ago and I'm looking for support trying to determine the root cause. MC is from a 2004 WRX.

MC had fluid topped off, 50-60% pad life all round with DTC60's. Fluid flushed with Motul RBF600 a few track events prior. Pedal went dead, straight to floor with no previous signs of brake fade prior to having nothing. Pumped a couple times but didn't really come back.

Testing out in the garage the brake pedal was firm without engine on/vacuum. The pedal was soft starting up engine and first time pumping brakes after failure. The brakes firmed up after a few pumps.

I took the MC apart and tried to diagnose. I noticed there was some fluid and debris collected around the fwd most (secondary?) grommet and there was minor wear on the secondary/fwd piston. With the MC off the car, the piston seemed difficult (seemed sticky/friction) to work in and out of the MC housing. After a few cycles the pistons seemed to compress into the housing without much difficulty. There is a ~1mm ring in the housing that has wear marks from being dirty. The locking cap and interface with the brake booster was also very dirty.

Thoughts? I am not 100% ruling out air entering at some point during a fluid flush, although unlikely. No noticeable leaks around brake lines. The fluid level in the tank seemed to have gone down by about ~2/10".






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Old 12-04-2018, 05:28 AM   #2
mrsaturn7085
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When you flushed the fluid, I bet you pumped the brake pedal using the buddy method for bleeding, didn't you? You over-extended the master cylinder piston travel causing the piston o-ring to travel over the ring of gunk that collects at the end of the normal, sealed-system travel distance. This is a common master cylinder failure, and most people only make this mistake once.

If you didn't do this... the problem is the same - the seal in the cylinder is shot and needs to be rebuilt or replaced. It happens.
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Old 12-04-2018, 04:27 PM   #3
car_freak85
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Quote:
Originally Posted by mrsaturn7085 View Post
When you flushed the fluid, I bet you pumped the brake pedal using the buddy method for bleeding, didn't you? You over-extended the master cylinder piston travel causing the piston o-ring to travel over the ring of gunk that collects at the end of the normal, sealed-system travel distance.
How do YOU avoid this? I've only ever had it be an issue on higher mileage MCs.

Do you just do half pumps on the pedal? Motive bleeder?
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Old 12-04-2018, 04:47 PM   #4
Charlie-III
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Quote:
Originally Posted by car_freak85 View Post
How do YOU avoid this? I've only ever had it be an issue on higher mileage MCs.

Do you just do half pumps on the pedal? Motive bleeder?
I can't answer for mrsaturn7085......for me, a 14yo set of seals, I assume I will have issues due to age and maybe water.
So.......new MC, bench bleed, bleed when installed, call it a day.

But that is just me.

Decades ago, you could get all sorts of hydraulic rebuild kits, now, most places don't have them and the price is close to a whole new unit you swap in.

Sheesh, I bought rear wheel cylinder rebuild kits for our 99 Impreza L wagon (rear drums), only to show my son how to do it.
For about $4 more, I could have replaced the pair of wheel cylinders.
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Old 12-04-2018, 06:22 PM   #5
corona672
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You can order the internal parts OEM from Subaru to rebuild. Since my MC housing is pretty beat I'll just be getting a new assembly.

I did use the buddy method, so I would concur that the seals in the piston could have over-traveled. Especially on 14 year old seals. Time to invest in a venturi vacuum or a power bleeder...

Last edited by corona672; 12-04-2018 at 08:21 PM. Reason: words
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Old 12-04-2018, 08:41 PM   #6
mrsaturn7085
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You do half pumps of the pedal to avoid the issue I described. You are correct that it is more common on higher mileage parts - the usual stopping point of the seal builds up a step of gunk over the years (it's like a little plow that just keeps building a pile of garbage) and it hardens and gets sharp enough to damage the seal itself. Having a wife or other non-car person pump the pedal without coaching them first is a sure way to destroy a master cylinder.

You don't need a pressure or suction bleeder - the buddy method is still the best and easiest to clean up... though speed bleeders work fine if you don't have any friends.
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Old 12-06-2018, 01:47 PM   #7
corona672
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Problem: My master cylinder broke!
Solution: Don't marry or hang out with non-car enthusiasts

Thanks for the diagnosis
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Old 12-06-2018, 03:01 PM   #8
mrsaturn7085
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I would not hone/rebuild a 14 year old cylinder.

You can have anyone pump the brakes, just explain the pedal travel thing before hand - easy enough.
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Old 12-06-2018, 04:50 PM   #9
car_freak85
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Quote:
Originally Posted by mrsaturn7085 View Post
I would not hone/rebuild a 14 year old cylinder.

You can have anyone pump the brakes, just explain the pedal travel thing before hand - easy enough.
Once you start playing with cars that are pushing 30 years old, you don't have much choice...
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Old 12-06-2018, 05:00 PM   #10
mrsaturn7085
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Subaru parts are like Lego blocks - I guarantee there are over a dozen valid part numbers that will work for this part if you're willing to use a line bender.
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Old 12-06-2018, 05:13 PM   #11
car_freak85
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Quote:
Originally Posted by mrsaturn7085 View Post
Subaru parts are like Lego blocks - I guarantee there are over a dozen valid part numbers that will work for this part if you're willing to use a line bender.
It's getting hard to find the 1-15/16" four-port MCs that were only available on base model 1992-95 non-ABS SVXs... (THE MC to mate big EJ brakes to an EA Subaru chassis).
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Old 12-06-2018, 07:37 PM   #12
mrsaturn7085
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I believe you mean 1-1/16" 4-port. I've never seen a nearly-2-inch master on a Subaru.

That's the same size as the STI master. Lines on the wrong side and don't want to bend lines? Euro and JDM masters have the port side flipped. Fix a 2-port cylinder by running two tee-fittings - you can still keep the diagonal split for safety.

I would put the extra effort into using new parts and a custom system than I would keeping an old master cylinder on life support.

Last edited by mrsaturn7085; 12-06-2018 at 07:43 PM.
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