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Old 04-30-07, 03:06 AM   #1
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Help: Brakes soft when car is running, yet ok when it's off?

I recently finished installing a new master brake cylinder in my 10th AE Rx-7, but I've found an issue with the brakes feeling very soft once the car starts up. I bled the brakes in the proper order from farthest to closest to the MBC, and made sure that the boost was emptied prior to bleeding the lines (as per my manual). After bleeding the brake lines I had a decent amount of resistance on the brake pedal (ie it felt like I had normal working brakes), but when I started the car up the brakes suddenly went very soft. After shutting the car off, I pressed on the brake once and it remained very soft, but the second time, and any time after that when I pressed the brake, it had normal resistance in it again until I started up the engine again. According to the checks in my manual the boost is apparently air-tight, so it's not a problem with a cracked hose as far as I can tell. And I'm not loosing any brake fluid as far as I can tell, so this has really got me confused as to what is going on. The original owner took it in prior to my buying it from him, and both places he took it to said it was the MBC.

What i'm wondering is has anyone run into similar problems with their brakes after bleeding them? Looking at some of the other threads I'm thinking it may be a leaky caliper or possibly a problem with the ABS, but I'm not too sure, and those threads didn't seem to match what I have going on exactly

Any help is greatly appreciated
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Old 04-30-07, 05:25 AM   #2
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Well I don't really know about the softness, but when the car is off the brakes will get very soft because they are power assisted and thus tough to operate without the engine making vacuum for your brake booster.
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Old 04-30-07, 08:34 AM   #3
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vacuum assisted brakes. when the car is off, the pedal will get harder because there is no vacuum present in the booster.
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Old 04-30-07, 09:19 AM   #4
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my guess is you had the same problem i did, a leak on one of the rear calipers where the e-brake cable goes into the caliper. when the car is parked with the e brake on (not running), the system is fully sealed, but when you go for a test drive, you release the e brake and the caliper starts to leak.
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Old 04-30-07, 10:45 AM   #5
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^ I had that same problem. Stupid design of the calipers with the push pin holding the spring which also holds everything else in. I had to weld that push pin on the caliper so it did not come loose again.

The brakes will get very soft when the car is running, what you don't want to have happen is the brakes going to the floor or have the car slowly slow down when you quickly press on the pedal. go for a short test drive, making sure your e-brake still works first, just to see if your brakes feel good.

Like others have said, when the car is off, the pedal will become very hard. This is normal. Because you have witnessed this, you can rule out a bad booster Which wouldn't make the pedal feel soft, only hard.
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Old 05-03-07, 08:48 PM   #6
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Thanks to everyone for the helpful responses. I'll be replacing the caliper this weekend so I will post what happens from there.
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Old 05-03-07, 08:48 PM
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