11 replies to this topic

[Diller]

I have a concern with my brake pedal that either just happened or I just noticed it. The car is a 94 and it has been a little over a month since I drove the car so my foot may be out of calibration. I drive a 1.6 to work and it has a much more immediate pedal pressure.

 

After rebuilding the calipers, I bled the system and with the car off I had a very firm pedal. As soon as I introduced vacuum with the car being on, there is about an inch drop before I get a hard pedal. I went ahead and bled the system some more while tapping on all the calipers to try to free up any hidden bubbles. I also checked the function of the vacuum check valve and it is working. After flushing a liter of brake fluid, same condition. For shiggles I replaced the master cylinder, took all the calipers off and checked for sticking sliders or pistons and put another liter of brake fluid through the system. Same symptoms. I will be at Road Atlanta this weekend and don't really have a way to test the braking performance before I get out on track. I guess I might try just locking up the brakes in the driveway to make sure I can get enough force on them without the pedal hitting the floor.

 

The only thing in the system that is not new is the brake booster however I figured I would get no assistance if that were the case. Everything may be fine but I wanted to compare notes with other to see if I am missing something.

[Erik Hardy]

I had the same symptoms at the beginning of the year with "not much life left carbotechs", new pads did the trick for me. No matter how much I bled everything, I could get a firm pedal with the car off but as soon as the car was on, it went to the floor. 

[Diller]

I had the same symptoms at the beginning of the year with "not much life left carbotechs", new pads did the trick for me. No matter how much I bled everything, I could get a firm pedal with the car off but as soon as the car was on, it went to the floor. 

 

It looks like the pads have plenty of life left but I have a spare set so I might as well throw those on and see if it helps.

 

Also, the rear calipers have had the e-brake adjuster removed so they shouldn't be contributing to the issue.

[bmarshall1]

Looking to see the results...

[Keith Novak]

Sounds about like my '95.  There is some travel before you can feel the pad making contact, and then Carbotech pads are kind of squishy.  The firm point is a bit below the gas pedal, but still quite a distance from the floor (unless I just drove over curbs or went farmin').  You can generally get a firmer pedal feel without the car running, but then as soon as you turn the key and the booster kicks in, it's a bit squishy again.  I've tried different hoses. I have East St. calipers, new master cylinder bench bled multiple times and everything new but the booster and the hard lines.

 

I modified my pedals and heel-toe technique to accommodate. I've been experimenting with different pads as well and they definitely have different feels for when they're firm.  If you don't bench bleed the master, you probably have bubbles in it and also bleed the brakes regularly.  You'll occasionally find a bubble.

[davew]

If you have used pads in the car, try a set of brand new pads. Any tapered wear will result in flexing of the caliper when the vac. assist is working. You won't notice it without the engine running.

 

dave

[Tom Hampton]

I see the same thing when the front pads get below 50% wear, which gets progressively longer as the pads continue to wear.  I've seen the same thing with both PFC97 and DTC-60 pads. 

 

I measure my pads as part of post-event inspection and do not measure any taper within the limits of my measuring devices (fore-aft, nor top-bottom).  I have only minor inner-outer wear differential (~0.010 for a 3 day weekend). 

 

I actually set the brake/gas pedal heights based on my minimum allowable on-track thickness. Once upon a time (before I did that) I had a couple scary moments on track when the pedal got long and I caught the gas-pedal in a threshold braking zone coming down from the top of 4th to a low-2nd gear haripin. I blew the braking zone the first time and ended up 4-off at a much elevated speed, and didn't know why. There was plenty of run-off, so no harm done (except to my pride). The same thing happened on the next lap, but I was more aware and realized  that I caught the throttle. I broke early that lap so, I was able to keep it on track.  After that, I elevated the brake pedal with spacers so that the two pedals were co-planar even in that configuration.  IIRC, brand new pads leaves the brake pedal ~1 inch higher than the throttle, which does not cause any heel-toe issues for me. 

[Erik Hardy]

I forgot to mention that my pads were also tapered pretty badly. They didn't look tapered when they were on the caliper but it was very apparent after I pulled them off.

[Diller]

I'll put on the new pads this afternoon and report back. I'll measure the pad taper as well. The pads I am using are DTC-60s front and rear. So far I have four race weekends on this set of pads and with as much meat is left on them, I thought I could get a whole season on them.

[Tom Hampton]

I'll put on the new pads this afternoon and report back. I'll measure the pad taper as well. The pads I am using are DTC-60s front and rear. So far I have four race weekends on this set of pads and with as much meat is left on them, I thought I could get a whole season on them.

You're easier on your DT-60's than I am, I guess.  I get about 3 race weekends including the test-day out of my DTC-60s. 

[Diller]

Ok I swapped in new pads and measured all the pads for taper. First off, the pedal is better. I wish it was stone hard without moving but I'll take what I can get. I drove into the shop at speed and was able to lock up all 4 tires. Besides being educational, it was fun and I have some nice tire streaks on the floor that add to the the decoration. Yeah I know, I should have had the go-pro running.

 

7 out of 8 pads had a taper of 0.5mm and one had a taper of 1mm. All pads measured between 10 and 11.5mm thick (including backing plate. They all have a bunch of meat still on them so I may try to find some shims to keep using them. I would attribute the taper to sticking slider pins and calipers that I just fixed. Some of the calipers were rather tough to get the piston out with compressed air.

[davew]

The biggest issue is getting the tapers mixed up. If the pads get a taper this way // and you put them back in this way^. The caliper will flex to make up the air gap when you push the pedal. Then completely retract as the pads push the piston back too far. Resulting in a long pedal.

 

Tom, I think you are getting kick back. learn to "test" the pedal before each corner.

 

dave