Strange Brakes

[Galwaylotus]

Strange one. I hadn't had my S3 out for a run for some weeks. As we have the annual WestRun coming up this weekend, I decided to blast out the cobwebs and polish the discs. I drove out six or seven miles getting up to speed and then applying the brakes hard. Repeated the cycle a few times and everything felt good. Then, returning home by a rough back road, the brake pedal suddenly went to the floor! I have a tandem master cylinder so that really took me by surprise. I reckoned I'd have to drive the last couple of miles (downhill) slowly and use the handbrake at junctions but then I pumped the brakes a couple of times and they came right back! The last mile or more everything was back to normal. When I got home I checked the master cylinder and found it still full, then looked at each disc and all looked fine - no sign of leakage or metal-on-metal.

Does anyone have any ideas?

[pereirac]

Do you have a servo? it could be leaking...the rubber diaphragm can split?

[Galwaylotus]

Nope. Converted a couple of years ago and ditched the servo. It had locked up on me so it was time to go to tandem master cylinder.

Current spec:
AP Racing 0.7-inch tandem master cylinder with remote reservoir.
Steel braided (Aeroquip) brake lines at all corners.
GreenStuff pads on standard front discs.
Original rear discs and pads.

Can't imagine the brake fluid boiled on the few hard applications I made.

[MickG]

How old is the brake fluid?
Sounds as though it could be a vapour lock.
Wouldn't hurt to do a fluid change none the less.

[AHM]

+1 for how old is the brake fluid!

[dpo#4]

Dear Galwaylotus,

With the observed absence of fluid loss, the likely possibilities are pad knockback due to hub freeplay, or master cylinder seal leakage. Seal lips are cupped toward the front of the car and may start to leak back to the reservoir. A sinking pedal is often more noticeable under light pressure, as heavy and abrupt application flares the seals tightly to the cylinder walls. The rough road could be a factor in pad knockback. In conjunction with a broken brake pedal return spring, a rough road and even slightly worn seals could cause the bouncing pedal to sink without any effort by the driver. Well, that's a deep one! I use my fingers on the pedal when pressing lightly as the hand's more sensitive. If there's no sinking and no hub freeplay, fresh brake fluid is always a good thing. If you ever disassemble the master cylinder, check the bore for wear at the areas repeatedly traversed by the pistons. Aluminum cylinders aren't worth rebuilding most of the time. Hope you find something definite. - Ray.

[worzel]

Hi Galway Lotus

I think pad knockback is likely. Years ago I owned a Scimitar Gte which had odd brakes. On the MoT rollers they showed excellent stopping ability and on the road they were the type of brakes you just forget about ie they did the job so well you never noticed them- but- if I had cause to put the car on full lock then immediately used the brakes the pedal sank to the floor. One application and normal pressure was restored. I never found out the cause and it mystified everybody I spoke to about it. Most suggested pad knockback but the front wheel bearings (same design as the elan) were exactly adjusted and the suspension was fully re-bushed etc- in any case the problem was present even before I overhauled the front end.

John

[simonknee]

+1 for knockback.

Check your disc runout and bearing play. New discs/bearings cured this for me a few years back.
Before this I had totally got used to anticipating a pre-brake pump to firm things up!
Not great from an emergency point of view

It may even just be the rough road jiggling them out!!

Simon

PS just realised I used the word "totally" in a sentence there. Way to Go!

[Galwaylotus]

Thanks, folks. I'm inclined toward the pad knock-back theory. I checked the pedal this evening and it is firm, as normal. There is no leakage at the pushrod nor at the calipers. The reservoir is still full and fluid is about two years old - probably could be changed but it's not what caused the problem. I pulled the front wheels and rotated the discs by hand. Nothing untoward there. Likewise the suspension and hubs do not have excessive play. All dry around the rear calipers too and all discs are "polished" by the pads so nothing seems to be hanging up either.

Although it's never occurred before I'm inclined to go on the run on Saturday even though I'll be leaving a bit more space to the car in front. No "last of the late brakers" on this trip!!

[Foxie]

You can fit anti-knockback springs underneath each piston. Lowest load best for a road car, available from Demon Tweeks

[simonknee]

Can you give a link to these Sean, the search on Demon Tweeks produces nothing.

[Foxie]

Can you give a link to these Sean, the search on Demon Tweeks produces nothing.

Here it is:http://www.apracing.com/Info.aspx?InfoID=110&ProductID=7

I got them from Demon Tweeks, but they are an AP Racing product.

The springs come in two diameters, under 35mm and over 35mm piston diameter, and in 4,7, & 9lb force.

I found the 4lb was strong enough for a road car.

[nigelrbfurness]

I've had this problem of the pedal going right to the floor with no loss of fluid before, on several cars and its always been the master cylinder - one of the piston seals seems to fail under pressure when its worn and maybe when the fluid gets hot. However it occurs, I've always managed to fix it by replacing the seals in the master cylinder or by fitting a brand new mc. I'd start there before looking at weird possibilities. Full travel of the mc piston rod seems unlikely to be possible even with very worn pads if thee hydraulic system is not leaking somewhere.

Nigel

[elangtv2000]

+1 on the mc seals, per nigelrbfurness - have had that happen to me, seen it on 2 other Elans.
Greg

[Galwaylotus]

Ok, folks. You're all wrong!

I went on the run today. The first leg was about 25 miles and I did my usual check of hand to the wheels/tyres. The left front wheel was too hot to leave my hand on it while the other three were cool. The caliper must have a seized piston and it's dragging constantly. After the stop we set off again - about 27 miles this time and when we got to our destination the wheel didn't feel quite as hot. We had been stuck in traffic a bit getting into town and that may have allowed the brakes to cool more. After a few hours in town, I made the more direct journey home, about 30 miles with no stops. When I got back to Galway the pedal went to the floor again! I nursed the car the last three and a half miles home with a very soft pedal and when I checked the wheel temps, the 3-eared nut was too hot to touch. I mean VERY hot!! The reservoir was still full. The pedal was virtually non-existent. After the car sat for about an hour, I went out and tried the pedal - fully back to normal!!

The problem is obviously brake fade from boiling fluid. The root cause is the seized caliper piston. I was planning to take off the wheels after the run anyway to get them stripped and painted while I await the new tyres I ordered so I guess I'll be doing a caliper (or two) rebuild while it's up on blocks!!