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Old 05-06-2015, 09:37 PM   #1
Loopey
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Member#: 296950
Join Date: Oct 2011
Location: Denver, Colorado
Vehicle:
2009 WRX
Black

Default Brakes failing while driving

I'm having a problem with my brakes failing to work while driving. I'm pretty sure that I have an issue with the check valve on the brake vacuum line, but given the seriousness and randomness of the issue I'm looking to see if anyone has any other ideas...

This has happed to me 3 times. Once today, and twice during the same drive last November. In the initial occurrence, it was -17F outside, so I figured that I just had a little moisture in my brake fluid that had frozen under the extreme conditions. It was also the cars first day driving after having been parked in the shop for 2 weeks. Today, it was a nice spring day, about 60F, and the car has been driven regularly.

Driving for 15-20 minutes on an open highway, with no need to use the brakes. Cruise control set at 75mph (that's the speed limit). Come up behind another car, and tap my brakes to slow down a little and disengage the cruise control before passing. When I tap the brakes, pedal doesn't move a bit. My immediate reaction is to try to pump the brakes, pedal still doesn't move. Then I push really hard on the pedal, and the brakes do work minimally, but the power boost is obviously not working at all. After pushing hard for 3-5 seconds, everything comes back into normal working order.

My normal commute has me driving this same highway twice a day, three times a week, for about 600 miles a week. I've only had the brakes go out on me 3 times total. As mentioned at the beginning of the post, I think there is some issue with the check valve so I plan to replace the vacuum line ASAP. I'm also going to flush & bleed the whole system (should have done that already, I know). Is there any reason that I should think it's the Master Cylinder, Booster, or something electrical?

Thanks in advance for any ideas or opinions-
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Old 05-07-2015, 03:26 AM   #2
Uncle Scotty
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OK buy Nates beans
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you need a mechanic
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Old 07-25-2015, 02:55 PM   #3
Boxer1687
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Member#: 369969
Join Date: Oct 2013
Chapter/Region: MWSOC
Location: Chicago, IL
Vehicle:
1998 Impreza Outback
Black

Default Brakes failing while driving

My vehicle.
98 Impreza outback sport
2.2. 5 speed.

Front disc brakes single Piston.
Rear drums

It started off by My passenger side front caliper was squeaking every time I reps see the brake pedal.
Thought is was the pins in the caliper and greased them.
And greased the piston. The squeak stopped after a week of being driven. (30 miles a week).
When it squeaked again I took a closer look and found out that the piston was leaking through the seal.

After a week I finally changed the caliper, (eBay ordered).
After installing it I test drove it and it locked up.

I bought another caliper and the same thing happened. So
My father had mentioned that the pads have brake fluid on them and they're wet so it could cause them to stick. So I changed the rotors with new pads and the problem is gone.
So I have an extra right caliper, I compared it and the only diffence is the mounting position of the brake line, and the bleeder. It is wise to change the driver caliper to the spare right side one? What kind of effect will that make?

Last edited by Boxer1687; 08-01-2015 at 10:09 AM.
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Old 07-25-2015, 10:51 PM   #4
mrsaturn7085
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Member#: 375462
Join Date: Dec 2013
Chapter/Region: NWIC
Location: Portland, OR
Vehicle:
2006 Impreza WRX STI
WR Blue

Default

You don't need a mechanic, but you have a vacuum or servo (brake booster, power assist, etc.) problem, not a brake problem.

Go somewhere safe, accelerate up to ~15-20 mph, shut the engine off and brake - it will be hard to stop, but you'll be able to. Move your seat forward so your butt/back is actually where it's supposed to be in the seat - you want leverage here. If you doubt your own strength, don't forget about the handbrake... assuming it's relatively in-adjustment.

If it feels the same as when you are "losing brakes", you either have a vacuum leak (rubber hose w/check valve) or you have a bad brake servo (booster).

Replace whichever you find to be faulty.

If it feels DIFFERENT when you try this, take it to a mechanic, as people on a forum can't 'feel' the brakes, and this is critical for this diagnosis.

Also, brake fluid typically freezes around -40 deg F. Your fluid isn't freezing.
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