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.

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!!!

[davep]

You must be braking into 5, then it is not far to turn 6 braking zone IIRC. It does sound like the pistons are being pushed too far back in that section. How hard are you braking in those areas?

[POS]

Our racing Mercedes did this and it was because the brake fluid was boiling.

[A.T.]

Hard braking into the bus stop. The next braking is going into the shoot after a long right hander. For sure I wasn't boiling the brake fluid. The brakes are big for this car, 2400 lbs. with cooling ducts. The pedal isn't mushy. It just goes to the floor and then comes back fully after one pump and then is solid for a full lap until the next time into that corner.

[chris_seven]

I used to Rally a Mk1 Escort Twin Cam - more years ago than I like to admit - that had a similar problem.

The pedal used to go flat to the floot but around once every hundred miles. It was a real nightmare and it persisted for aboyt about 9 months, the rest of the time the pedal was perfect.

I used to dab the brakes early all of the time and it ruined our overall performance.

During the 9 months we changed everything, calipers, master cylinders, hubs, fluid, discs, bearings - nothing worked.

Finally we changed out all of the hardlines and never had another problem.

I was told, but can't conform that a damaged flare can allow air in but not let fluid out.

I find it hard to believe but changing hardlines did work.

[A.T.]

Thanks, Chris. I'll run that by my mechanic and see what he thinks.

Escort? Great car. Any pictures?

A.T.

[Mr9146]

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.

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!!!

I am going to jump on the pistons/pads too far back band wagon, but I think it you may have an issue with a warped rotor or rotors.

Although, the fact that it only happens on one corner/braking zone of the circuit it is a bit of a mystery.

But, if you're looking at "everything" don't rule out warped rotors. A wobbly rotor will push the rotors and pistons back to the point that a double-pump is required ... this happened on my father's daily driven 75 Carrera with Turbo brakes in the front. The rotors were worn out and warped easily, and after a few stops they would warp and wobble enough to knock the pads and pistons back into the calipers. If the braking zone is after a long right hander then I'd suggest a look at the LF rotor first. Are the rotors floaters?

[A.T.]

Well, I think we may have this figured out. I'm sorry I can't remember who or where the likely answer was given to me but it was by someone who seemed to have extensive racing experience. This wise person asked me if I'd been going over any curbs or rumble strip just before the corner where I was having the issue. That was in fact exactly the scenario. Their are massive FIA style curbs going through the bus stop at Watkins Glen and I was using them. And that is the only spot on the track where I was. The next braking point is the offending corner. The theory is that the vibrations from bouncing over the alligators is slapping the pads and brake pistons back far enough that it takes two pumps of the pedal to bring the pads up against the rotors again. This gentlemen suggested that this was quite common with track cars and that a little prod to the brake pedal well before each serious brake zone would bring the pedal back up.

This sounded reasonable and answered a question I'd had about the driver in this phenomenal in car video.

http://www.youtube.com/watch?v=MMpWi1iO-_I

I got that he was using left foot braking in the corners but why was he stabbing the brake pedal down the straight after Eau Rouge? Ah haaaa!

None the less I think the old 911 is ready for a caliper rebuild. It may well be that things are also a little more worn and sloppy than ideal.

Hope this helps others with similar issues.

Cheers.

[jameshtaylor]

Have you tested your theory yet? We'd all love to have confirmation. Probably you would too!

[A.T.]

Hey James

No tests yet. It just happened at the last event of last season. The caliper rebuild is scheduled for a few weeks from now. Also, it only happened at Watkins Glen in that one corner. We don't have those alligator style curbs at Mosport. Or, we didn't. Not sure what things will look like this year. They've done some major renovations and repaved a few sections. We may be getting a NASCAR event here this fall, like the Glen event. Thus the upgrades. Maybe we'll have some new curbs.

I'll update once I'm on the track again.

A.T.