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Discussion Starter #1
Looking for some help on what to look at. Here's the situation: First brake pedal press feels not so firm, second and consequent presses are very firm and brakes are more responsive. If I let the brakes unused for a while and press again, it becomes not so firm again. Basically if I press the pedal within like 30 secs each time it will remain firm. IF I let go for 2-3 minutes it becomes the mushy-not-so-firm.

I started wondering if there's air in the brake calipers since I rebuilt them recently, but I've bled around 500ml (yes, half a litre) per caliper without any air coming out. First 4-5 pumps did get air out, but afterwards it was pure brake fluid.

There's nothing wrong with the caliper rebuilt since it's a fairly simple task (only get to change a seal and some rubber guides) and this actually should had improved the brake pedal feel.

Any ideas what to look?
 

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If your confident on everything else you've done so far, it sounds like a failing master cylinder to me.


Any other brake guys please chime in.
 

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Para,

After my brakes were bled at my shop, and I was helping pump, when it was all done, before the car was started, I was told to pump up the brakes really firmly...before starting the car.

I asked about it and was told it's to build up pressure. I don't understand the reasons beyond that...but maybe it's as simple as that if you didn't do this?

Also, I wonder if maybe it has something to do with the brake booster line coming from your upper intake boot. You notice any issues with the connection or the hose? Probably not, but I'm not a brake expert.

Oh, also, I know that when redoing calipers, that some diys say to fill the caliper with fluid first, since there are some spots in the caliper where it's hard to get the air out of, so maybe it's also just a matter of some more bleeding. Easy enough to do before replacing parts like master cylinder.

Sorry they're all guesses.

Bump!
 

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Is this while engine is running or engine is off?
Looking for some help on what to look at. Here's the situation: First brake pedal press feels not so firm, second and consequent presses are very firm and brakes are more responsive. If I let the brakes unused for a while and press again, it becomes not so firm again. Basically if I press the pedal within like 30 secs each time it will remain firm. IF I let go for 2-3 minutes it becomes the mushy-not-so-firm.

I started wondering if there's air in the brake calipers since I rebuilt them recently, but I've bled around 500ml (yes, half a litre) per caliper without any air coming out. First 4-5 pumps did get air out, but afterwards it was pure brake fluid.

There's nothing wrong with the caliper rebuilt since it's a fairly simple task (only get to change a seal and some rubber guides) and this actually should had improved the brake pedal feel.

Any ideas what to look?
 

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Discussion Starter #6 (Edited)
If your confident on everything else you've done so far, it sounds like a failing master cylinder to me.


Any other brake guys please chime in.
I did one mistake; I let the brake fluid reservoir go empty (or almost empty) while I was changing the seals (fluid was dripping from brake lines inside the wheel well. Could I have introduced air inside the brake master cylinder?

Para,

After my brakes were bled at my shop, and I was helping pump, when it was all done, before the car was started, I was told to pump up the brakes really firmly...before starting the car.

I asked about it and was told it's to build up pressure. I don't understand the reasons beyond that...but maybe it's as simple as that if you didn't do this?

Also, I wonder if maybe it has something to do with the brake booster line coming from your upper intake boot. You notice any issues with the connection or the hose? Probably not, but I'm not a brake expert.

Oh, also, I know that when redoing calipers, that some diys say to fill the caliper with fluid first, since there are some spots in the caliper where it's hard to get the air out of, so maybe it's also just a matter of some more bleeding. Easy enough to do before replacing parts like master cylinder.

Sorry they're all guesses.

Bump!
By design, the bleed valve is located on the top part of the caliper therefore all air should come out. I also gave both calipers some taps with a rubber mallet to free any remaining air bubbles. Brake booster line seems fine. Will do another bleed session..

There is a Minor Leak (Rebuild Calipers) or Master Cylinder is going out.


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Checked both calipers after 1 week of testing and they are both dry, not even humid around the seals. Could my mistake described above contribute to the bad braking I'm experiencing? If so, how can I bleed the master brake cylinder?


Is this while engine is running or engine is off?
Probably while it's off
 

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Yes, you could have introduced air into the system if it went dry.

Now it's time for a power bleeder & some ATE Super Blue brake fluid.
 

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Here's how my brakes feel when I turn the car off. At first there is still boost (vacuum) in the brake booster, so I can press the pedal down far and it feels soft. After pumping it a few times, the vacuum is gone (because engine is not running to create more) and the pedal gets very firm. Once I turn the engine on again, pedal gets "soft" because I have brake boost. AFAIK, this is normal when the engine is off.

If this happened while driving and stopping at slow speeds, you should have brake boost all the time, so having the pedal be soft when you first try to stop then firmer on the 2nd press would suggest a problem, because the pedal should feel the same all the time, unless you've boiled your brake fluid or overheated your pads at the track. You could test my theory by disconnecting your brake booster line temporarily. Then start the engine. The brake pedal should feel firm all the time and be the same with motor running or not, because there is no longer any brake boost. Don't drive around like this a lot because it might create a lean condition.

If you have air trapped in the brake lines or calipers, then the pedal should feel a little mushy even after you've pumped it a few times when the engine is off, because you're compressing the air. If you have air in the ABS system... I don't know what that feels like. If you left a bleeder screw open, or a brake line disconnected, or have a big leak in a caliper, the pedal should go all the way to the floor each time and then you find a puddle of brake fluid under one wheel. (I have done that!)
 

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Discussion Starter #9
Yes, you could have introduced air into the system if it went dry.

Now it's time for a power bleeder & some ATE Super Blue brake fluid.
I assume a power bleeder is the way to bleed air in the brake master cylinder and not repeated 2-person bleed jobs right?

Here's how my brakes feel when I turn the car off. At first there is still boost (vacuum) in the brake booster, so I can press the pedal down far and it feels soft. After pumping it a few times, the vacuum is gone (because engine is not running to create more) and the pedal gets very firm. Once I turn the engine on again, pedal gets "soft" because I have brake boost. AFAIK, this is normal when the engine is off.

If this happened while driving and stopping at slow speeds, you should have brake boost all the time, so having the pedal be soft when you first try to stop then firmer on the 2nd press would suggest a problem, because the pedal should feel the same all the time, unless you've boiled your brake fluid or overheated your pads at the track. You could test my theory by disconnecting your brake booster line temporarily. Then start the engine. The brake pedal should feel firm all the time and be the same with motor running or not, because there is no longer any brake boost. Don't drive around like this a lot because it might create a lean condition.

If you have air trapped in the brake lines or calipers, then the pedal should feel a little mushy even after you've pumped it a few times when the engine is off, because you're compressing the air. If you have air in the ABS system... I don't know what that feels like. If you left a bleeder screw open, or a brake line disconnected, or have a big leak in a caliper, the pedal should go all the way to the floor each time and then you find a puddle of brake fluid under one wheel. (I have done that!)
Yup! correct on the above, however I can safely eliminate fluid leaks. Correct again if there was air within the caliper, the 2nd attempt to brake would yield the same results. I think they all point to the brake master cylinder. It either coincidentally failed or I have introduced air in it. :eek:

I'll need to consult ESS before disconnecting any lines though
 

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Discussion Starter #10
Another observation (while waiting the power bleeder to arrive), the issue is much more amplified when I am descending a hill and I am engine braking the car (no throttle). The pedal feel returns to the same problematic condition much faster than when engine is under load/accelerating. Does this mean anything at all ?
 

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Discussion Starter #11
Solved

Apparently, I solved this problem. With the engine running but car stopped, I pressed the brake pedal as far as it could go a lot of times, maybe 20 or more. During the last presses the brake pedal became "normal" and the mushy feel gone.

Since then it has been fine, as I did this almost 3 weeks ago :evil:
 

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Interesting... that suggests you either pushed some trapped air out of something (master cylinder?) or fixed a leaky seal. Or maybe smoothed out the interior of the MC? Don't know what to say, but glad you fixed it.
 
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