Squishy Brake Pedal at the track. Suggestions ?

[RockyTop]

When last at RoadAtlanta I had just had my tech replace the brake fluid with Motul 600 (bottles still sealed) and he supposedly bled the brakes. I got a squishy pedal on 2-3 occasions though I was not driving or braking the car nearly as hard as I have in the past on hotter days. The only changes to the car since the last time were softer spring rates (which were still higher than stock by about 10%). I assumed that I was just boiling the fluid. I thought that that perhaps I was easing into the brake instead of stabbing them as is appropriate for threshold braking and therefore was putting alot more heat in them due to my granny like braking practice. I then changed to a more aggressive initial braking application (and admittedly deeper into the braking zone), but saw no improvement. They would pump back up fairly shorlty thereafter, but it sure destoys your confidence.


For those of you who have driven there, it at the end of the braking zone leading into turnm one (kinda hairy/scary) and was no doubt due to the heat built up there and from the immediately preceeding long brake zone at the end of the 5000 ft. straight just before Turns 10a and 10b.

I have not realy checked my brakes since then as they have been working fine on the street. FYI I have Alcon fronts; stock rears; O-ring removed. Perhaps there could be a leaking seal or something? Any suggestions as to alternative causes ?

 

[Chuck 98 RT/10]

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[GTS Dean]

Rocky,

1) Be sure that the proportioning section is bled completely.

2) I changed fluid on my '93 one time, but let the reservoir get too low and accidentally got a bubble of air into the master cylinder. After scaring myself half to death about 3 times on the track, a friend who races IMCA dirt cars suggested bleeding the M.C. We did so, and the problem went away!

 

[RockyTop]

Thanks Jon, Dean and everyone else. Jon, my pads were Porterfields (the race versions; R-4s ?). I have braided brake lines, so they should not have been the problem. It definitelty wasn't pad fade so I suspect Dean is on to something. I remember looking at the reservoir for the master cylinder and it looked a little low so that is certainly possible.

As far as the O-ring, my Viper tech performed it (and threw away the O-ring Darn it !). Jon, How is this procedure typically botched ?

 

[PMUM]

Did you ever consider that with those Alcons (6-pistons right?) you are displacing a lot more fluid and the stock master cylinder isn't adequate anymore? I put bigger brakes on my Honda and had the same problem. The only solution was a bigger master cylinder. The problem was solved.

 

[GTS Dean]

Paul,

You make a very good point. However, the normal travel of brake pistons is measured in thousandths of an inch. Even if you have 14, or 20 pistons instead of 10, the stock MC should handle the fluid displacement requirements - FOR A WHILE. More pistons lead to volume problems as the pads wear. If you run a very soft pad, it is possible to go through 60mm (2x15mm per caliper x 2 calipers) over a weekend. That is a lot of volume in piston travel!

To maintain enough working fluid and make up for pad wear with increased piston area, you need to keep an eye on the reservoir between sessions and add make-up volume to compensate. Conversely, when you retract the pistons and install new pads, the fluid is returned to the MC and could overflow.

Like Winnie the Pooh said: "Think. Think, think, think."

Dean

 

[PMUM]

Dean, I strongly disagree. I knew nothing about brakes until I roasted the S2000 brakes into oblivion at Laguna Seca. Unfortunately, nobody at the time (January) made a front AND rear brake kit (the rear rotor is solid--big problem). Anyway, we had to fabricate our own kit. Once the 4 piston CNC calipers were hung on all four corners (from 2 piston front, 1 piston rears) and it was bled, the pedal went almost straight to the floor before coming in hard.

Step-2--increase the size of the master. This helped but the pedal was still VERY soft. Unfortunately, the piston displacement percentage went up more than the master.

Step 3--go to dual masters. This was the only way to push enough fluid to keep the pedal relatively hard.

Most cars have residual valves (or the ABS does it) that keep light pressure on the pistons so the pads don't fully retract. If they do (piston slap) then you get a very inconsistent pedal. When the rotors are slightly warped you get this and the pedal goes down further. So it shouldn't matter much how much pad you've got left because the pistons shouldn't travel further. A small amount of pad will cause the level in the reservoir to go down, but the pedal doesn't change. When the pads are old in my Viper the pedal doesn't go down any further.

Obviously, you've gotta find out the change in area of the pistons. I think stock fronts have 2 38mm and 2 42mm pistons (don't quote me on that). You need to find out how big the pistons are in the Alcon kit. Unless they are 3 very small pistons I'm sure it's displacing more fluid.

I'm certainly no engineer, I just play one on the internet. I'm sure somebody will come in here and give us all a lesson in fluid dynamics, but I do know that last month when we went from 6 total pistons on the S2000 to 16 the pedal was gone. Going to a bigger master brought it back up, and finally going to 2 masters brought it back to where it should be. That's why when Sean Roe puts 6 pistons calipers on the front and 2 piston calipers on the rear he uses a bigger master.

Hope this helps.