Brake Pedal Hits Bottom???

During the last two events at the track, I have noticed a different feel in the brake pedal but I don't know if it results from pad and tire changes.
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In the past, with AVS Sports and stock pads, I could lock up the wheels as I went a bit past threshold braking. With threshold braking, I could hear the tires chirping.
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Since going with Kevlar pads and a R rated tires (Proxies), I don't seem capable of locking the wheels. In fact, when I apply the brakes hard, I seem to "hit bottom" - - this is I can feel a definite stop to the brake pedal, but the pedal is NOT on the floor and I am not experiencing fad - the feeling is there from the second lap to the last lap on the track. I only hit this "stop" when using maximum braking, (Turn 1 at Summit, end of the straight, braking from 125 to about 35). I just hit the "stop" and wait for the brakes to "come on" and they do.
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Today, sitting at a traffic light, I tried the brakes - I get considerable resistence on the pedal, but if I really stand on the pedal, I hit "the stop" with an audible mechanical sound, a clink.
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The brakes were bled before both events and the fluid (ATE Blue) is about 3 months old. The master cylinder was rebuild last fall, as were the front calipers when we were flushing out the remaining DOT 5 silicon crap.
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I don't think the clinking sound is normal and I don't think I should be hitting a mechanical "stop" in the brake pedal. Ideas???????
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As usual, much thanks for your thoughts and help.
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Chuck

Could be many things Chuck, air in system etc.
But sounds like a problem with the power boost for your brakes
The way to check to see if its working is hold your foot on the brake pedal while starting car, after a few seconds you should feel the pedal pull back some and get firm

[This message has been edited by jryerson (edited 07-15-2000).]

Cylinders, like master cylinders, sometimes get a lip inside them due to repeated piston travel. Then, if the piston is ever asked to move farther, it will click or clunk over the lip. You've recently rebuilt yours, but this still could be happening. FWIW.

I'm thrift-minded (cheap) when it comes to car parts, but I've had bad experiences with rebuilt brake master cylinders. And rebuilt water pumps especially, by the way. I would never put a rebuilt water pump on a 911. This I can say with certainty.

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'83 SC

Water pump?

What freakin water pump?

Superman - -
I went back and checked my receipts last evening - in fact, the master cylinder was replaced last August, not rebuilt. The calipers were rebuilt.
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However, just before going to the racing pads, I did fry a new set of street pads on the track - they had not been bedded properly. By the third consectutive day on the track, I was getting splotches on the rotors. Could the piston in the new master cylinder travelled too far and thus now need replacement?
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Chuck

Chuck:

Look at your e.mails. I responded to your DC Automotive e.mail solicitation, and responded that I thought it was a matter of your Kevlar brakes taking a while to heat up before they actually stopped your car. Since your car seems to actually slow down or stop at some point, it is not as if there is an actual physical "stop" that is being encountered, but rather a time-frame delay from when you apply the brakes, then the pads touch the rotors, and there is THAT "stop", after which they brakes slow you down.

Can this be the reason you feel a "stop"?......Andras

LOL @ Superman You crazy, boah!

You know, you could put a rebuilt water pump in a 911. It would just have to be in the box so you don't get your seats messed up.

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Mark Szabo
1986 911 Targa 3.2

Andras - thanks for your email - I just replied.
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What you say makes sense except for the fact that while sitting stopped at traffic lights, I can depress the brake pedal all the way down, through increasing pedal resistance, and hit a mechanical stop and accompanying clink. It's the same "stop" I hit on the track.
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Another lister suggested that I might be hitting the maximum piston travel in the rear circuit of the master brake cylinder. This might be the case.
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Chuck

Well, I made the comment about the lip that forms in a MC after many years of use, but I question whether this would be your problem, given the new unit. Might be, but I wouldn't think so. Same symptom though.

Different pads should not change the MC piston travel, by the way. Typically, if a MC piston is travelling over that 'wear lip,' it is because of sponginess (water, air) in the system. But that doesn't sound like your problem either.

I'm not very helpful at times I'm afraid.

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'83 SC

Hey Superman - -
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You might still be on the right track - that other listed also mentioned that the cause of excessive rear circuit MC piston travel might be air in the MC. He questioned if the MC was bench bled when installed. (I don't know)
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Question: I just received a Power Bleeder and was planning to rebleed the brakes myself, following the lengthy and repetitive procedure described in the PP tech pub. Will this procedure get rid of any air in the MC or is another step required?
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Chuck

I've hit that stop as well. I was concerned for a while but realized that it happens when I'm asking a lot out of my brakes prior to them heating up. You have great tires and it will be more difficult to lock up the brakes. I have the SO 2's and I never lock them up on the street. And after a few hard stops, I don't hit that stop.
Racing demands a lot of your system. I've thought about getting steel braided lines. I do believe that we are getting a ton of flex when hitting the brakes that hard.
Can you relate to any of this? Let me know...
Kevin