Brake Pedal Scare

[A.T.]

Okay. I've got a real mystery for all mechanically inclined.

I was running in a PCA DE at Watkins Glen in my '72 RS replica. I'm lapping comfortably, building my tempo. I'm approaching the Bus Stop. Everything is good. I'm going around the long right hand sweeper, the Carrousel. Everything is still good. I'm diving down the Chute into turn 6 (down hill banked left hander) and the brake pedal goes to the floor. But just once. On the next pump the pedal is back. At first I thought I'd missed the pedal all together. I kept going without issue. Next time around in exactly the same spot it happens again. The pedal goes to the floor again and is back on the second pump.

What da fu...? Has anyone ever had the same experience?

I bled the brakes at the end of the day but it did it again the next day. I continued on, making sure I gave the brake pedal a little stab with my left foot before fully commitiing to all the more serious braking zones. Most of the way around I was fine. But everytime into 6 if I didn't do a full left foot pump it would go to the floor again.

I asked a few guys at the track and all suggested a brake bleed and then suspected the master cylinder. I bled the brakes thoroughly. If it had been air in the lines I'd have had the problem all the way around. No?

The only other suggestion was that I may have a bad or loose front wheel bearing allowing the rotor to push back the brake pads and caliper pistons, displacing enough brake fluid from the caliper to require two pumps to move the pistons and pads all the way to the rotor again. It sounded like a stretch to me but when I got home my mechanic suggested the same thing. Then we checked the front wheel bearings and hubs. Everything was fine.

I'm using Brembo calipers all around with Brembo floating rotors on the front and 930 rotors in the back. The master cylinder is the only thing we haven't looked at yet.

One suggesting from someone on the Eaqrly S Registry was to swap out hard lines, that a damaged flare can let in air without letting out fluid.

I sure would love to hear some thoughts on this from anyone who has some.

Thanks guys.

A.T.

P.S. Is there ever a time when everything is working on these cars, some kind of mechanical plateau free of overlapping issues? Jeez!!!

[pete3799]

What brake fluid are you using? Is it possible that you're boiling your fluid before you get to turn 6?

[A.T.]

Thanks, Pete.

It's unlikely. It was super cold that day. I've got brake ducts and the brakes are huge for that car. I use the fancy blue stuff for fluid and swap it regularly.

I don't think braking into the bus stop is any worse than going down into turn 1.

Also I've never had the problem anywhere else, not even going into turn 5 at Mosport on a brutaly hot day.

A.T.

[ShakinJoe]

Scary and weird.

No fluid leaking?
Speed bleeders?
Did you purge any air on the bleed process?
Have you changed fluid type recently?
Is MC making any weird sucking noises under compression?

[Joe Bob]

Heat is cumulative....how long were you on the track?

Also, older soft lines if using rubber oem can swell inside. Braided lines should be replaced annually.

[DRACO A5OG]

Which Fluid did you use? sounds like it boiled.

You may want to consider the air damn/divertors to keep those caliper's cool. They just clamp on

[LM3929]

How much slop in the frt wheel brgs?

Lorne m.

[Dave Colangelo]

Strange that it is always in the spot on the track, have you considered flushing the system with mineral oil then putting in fresh fluid. I would take a look at the master cylinder and consider replacing the break lines. I dont know much about the fluid boiling so I cant say much to that.

Regards
Dave

[tonythetarga]

Originally Posted by LM3929

How much slop in the frt wheel brgs?

Lorne m.

I think Lorne is on the right track. I know WGI very well. The Carousel is loading up the left side the whole way. If you have a little extra play on the wheel bearing or a slight warp in the rotor you will push the caliper piston back in the caliper and have a lot of travel to get it back up when you need it (and you really need it) for the braking zone before turn 6. I'd be checking front wheel bearings for play.

[cashman]

Do you go hard over gators or bumpy road before that turn? It may vibrate the pistons back a little. I have a turn at my local track that does something similar. The pedal doesn't go to the floor but it does travel further.

[Mr9146]

Rotors could be warped. See my answer on the other place.

[JP911]

Could be pad knock back. If you do a search in the 911 tech forum and the autocross/racing forum you'll find some discussion. Basically, vibration and distortion causes the brake pad to force the pistons back into the calipers, leading to the exact experience (with the pedal) that you describe. I recall reading that it tends to happen with Boxster brakes on 911s.

[Eagledriver]

Pad kickback, as described above. I've gotten to the point of giving the pedal a pump before each braking zone. If you watch the pros race, they do that all the time. You get used to it and it becomes second nature.

-Andy

[Tippy]

Could be pad knock back. If you do a search in the 911 tech forum and the autocross/racing forum you'll find some discussion. Basically, vibration and distortion causes the brake pad to force the pistons back into the calipers, leading to the exact experience (with the pedal) that you describe. I recall reading that it tends to happen with Boxster brakes on 911s.

Learn sumthin' everyday!

[A.T.]

Hey Guys

Wow! Thanks so much for the great feedback.

Okay. I'm using the fancy blue brake fluid. It's fresh. The front rotors are Brembo floating and the rear are 930. Both are new this season. I'm using mild Hawk pads. We've checked the bearings and hubs. No issues there. The bearings are also new this season.

I think Cashman and Andy are on the right path.

Cashman, as a matter of fact I was kinda pounding it over the gators through the Bus Stop. That's the only place I do that. And my mechanic suggested it was probably time for a caliper rebuild anyway. He mentioned something about the seals being worn and loose.

Andy, the pedal pump thing is pretty much what I ended up doing for the rest of the event. Here's an awesome in car video of a guy in an early 911 at Spa doing exactly that (plus a whole lot of left foot braking too) just before Eau Rouge and again at 1:03 min and 1:05 min. Now I understand what he was doing. In the corners was obvious but down the straights? Now I get it.

http://www.youtube.com/watch?v=BrePIvGjRes

Thanks again all.

I'll run this past my mechanic.

Cheers

A.T.

[FPB111]

I suspect knock back from your floating rotors over the turtles/curb with lateral Gs in the bus stop turns.
Hard braking at the end of the straight, then bump right curb, then left, left, right, curbs, then hard acceleration on right turn into the chute down into the boot and hard...WTF??? brakes at down hill left.

Tighten up the spacing between your "T nuts" (or whatever your setup uses to allow float) to cut down on lateral movement of the rotors. Floating rotors really only want to expand radially to allow growth in dia from the heat. I am assuming you are using a 23mm master cyl.

I ran 917/930 calipers & floaters on my 1972 for 17 years.
Initially used Porsche aluminum carriers. They wear quickly.

Then went to VCI aluminum carriers with steel? "T" nut/spacers. My preference was to machine material off a few of the "T"s, evenly spaced around the rotor (bellville washers will do the trick also), to get a snug fit to reduce lateral float and knock back.

I eventually went back to the Porsche alumunum carriers and machined the nubs off the carrier (Porsche has a carrier with square nubs for the rotor to ride on and an outer ring to capture the rotor) so that I could machine brass bushings that lasted longer and I could control the dimensions and replace worn bushings for pennies in stead of Franklins.
YMMV

[djpateman]

Originally Posted by A.T.

Cashman, as a matter of fact I was kinda pounding it over the gators through the Bus Stop. That's the only place I do that.

It sounds like this is the cause. One unique event followed by another.