Warren928

I was driving recently for about 3 hours in this 15 degree weather, on my way home and need to slow down for a hairpin corner. I apply the brakes and no stopping power at all to all four wheels, the pedal goes straight to the floor, could have sworn I heard a crackling noise at that moment. Entered a 15 mph corner at 35-40, and I had to correct just after the corner with some opposite lock, but all is okay.
Funny thing is that the car didn't do that again after or before the incident. Havent driven her since. And no, I wasn't going hard on the brakes prior or being speed racer, just cruising along.

Any ideas on what that may be? My first thoughts are:

1) Booster unit has start of a problem in diaphram
2) Vacuum line to booster unit has failed.

Gretch

Neither of those issues should cause the pedal to go to the floor.......either your brake fluid system leaks, or it does not. That is the second thing to check out. First, Look at the brake pedal where it connects to the master cylinder rod.....

I know if that happened to me, I would need to recondition the leather seats, at least the driver's seat anyway......

JET951

we have had a few cars do this at our work and it has always been the master cylinder. the seals leak internally and the fluid returns to the bottle.

FBIII

You might check to see that all your wheel bearings are within spec. If your wheel bearings are loose they could kick back your brake pads within your calipers and then the first time you hit the brakes you are repositioning the pads closer to the rotors before they take hold. Had something like this happen at Lime Rock. Somewhere between big bend and the back straight I would loose my front brakes every lap. I had to pump the brake pedal once or twice down the back straight to guarantee having all my brakes at the end of the back straight. Scared the crap out of me the first time it happened when I only had the rear brakes and the pedal went almost to the floor.

Brett928S2

Hi Warren,

It is NOT the booster as you Americans call it (we Brits call it the brake servo) if it was the booster OR a vacuum problem you WOULD STILL HAVE SOME BRAKES....boosters are brake ASSISTORS to save your leg muscles they can fail,,which ALWAYS leaves you NORMAL braking although your leg has a LOT more work to do...
So...
Your problem is almost certainly the master cylinders seals gone
Although I am MECHANIC I have not actually looked at the specs of a 928 M/C but.....most m/cs have a DUAL seal system...
That means that the first seal is used for normal braking and the SECOND seal SHOULD stop whats happening to you I suspect the first AND second seal are faulty now....

AND THEY WILL FAIL AGAIN !!!!!!!!!!!!!!!!!

Please DO NOT drive the car ANYWHERE until you have had the Master cylinder checked or changed...


Hope this helps Brett

928 S2 AUTO V8 4.7 LTR 1986 IRIS METALLIC BLUE WITH PROMAX CHIPS, RMB, KICKDOWN SWITCH.

Robert_H

When my brakes went out on my car, the pedal also went to the floor. I could build up some pressure by pumping the pedal, but within a minute the pedal went slack again.

With mine, it turned out to be two small leaks in the piston seals in the front calipers. Both of the leaks were small enough that you could not see any leakage from outside the calipers.

Rob

Warren928

Thanks guys.
The other thing was that normally I dont see a car out here but every 2 minutes, BUT of course there were 5 cars right there when it happened today. Also a scenario almost identical happened in a different car too. I just somehow have managed not to crash in both instances or go in the lake!


I think the wheel bearing are tight, but that is a good point.

I am going to check for fluid loss. That will determine if its the master cylinder or not, I think. If its peeing fluid, then still I cant imagine it being so intermittent, but you never can be too sure.

Bill51sdr

Considering the temperature you MAY have hit some black ice, and since your 86.5 has ABS... I believe your brakes acted as programmed to deal with ICE. This has always been a downfall of ABS. Just my .02...

Rich9928p

The brake pedal doesn't go to the floor when ABS activates! ABS cannot provide more pressure to the calipers than what the master cylinder provides - the three modes of operation to control wheel lock-up are:

- hold pressure
- decrease pressure
- increase pressure (to the available pressure from the master cylinder)

This is why you should stomp on the brake pedal as hard as possible if you encounter an ABS situation - the more pressure the more braking pressure the ABS has to work with. If ABS needs less pressure, it bleeds brake fluid out until the desired pressure is reached.

If the brake pedal sinks to the floor, there is a fault in the system either from a leak or from a bad master cylinder.

Brett928S2

I am going to check for fluid loss. That will determine if its the master cylinder or not, I think. If its peeing fluid, then still I cant imagine it being so intermittent, but you never can be too sure.[/QUOTE]

Hi again,,

You may have no fluid loss ,it CAN leak into booster....its definitely a hydraulic fluid/seal problem,,I have seen too many not to know...and id put money on it being Master Cylinder but it may be at the wheels although i doubt it

Hope this helps Brett

928 S2 AUTO V8 4.7 LTR 1986 IRIS METALLIC BLUE WITH PROMAX CHIPS, RMB, KICKDOWN SWITCH.

Bill51sdr

Ok, I stand corrected... thanks Rich. I do have to ask this, are you sure the pedal went to the floor? I only ask because usually when you have a pedal go to the floor it takes a pump or two to get back hydraulic pressure and you said nothing about having to pump the pedal in subsequent attempts to stop. If you did encounter some ice before that turn you would have no sensation of braking at all, no matter how hard you step on the pedal, the ABS will keep the wheels rolling. The crack you heard may have been the ABS system working, it's pretty noisy when working on these cars. The current editor of Car & Driver, Csaba Csere had a situation about 20 years ago in an ABS equipped Audi where he was on a Freeway and encountered a Semi stopped in the lanes ahead of him. He stepped on the brake pedal and nothing happened, turns out he was on black ice. He had to turn the car sideways in order to slow it and hit the trailer as his car was sliding sideways, severely injuring his wife. You may well have a mechanical problem, but just wanted to bring this up as a possibility in case no cause is found for your problem. Keep us posted.

FlyingDog

In the decrease pressure mode, I would assume the pedal would drop, although it should only do so for an extremely short period before going back to hold or increase pressure.

I've had a lot of problems with a soft (sometimes non-existant pressure) pedal. I pulled the ABS relay and the brakes appear to work normally now. I'd recommend going through all the brake hydraulics and booster tests. If none of those indicate what the problem is, try pulling the ABS relay or fuse and do some hard stops on dry ground.

rixter

even when you weren't driving!

FlyingDog

At least I was in nice comfy sport seats then.

jpitman2

M/cyl seals operate on the principle that when there is pressure in the V shaped groove the seal lip is forced out to the walls and provides a very good seal. When they get old and worn, the edges of their lips get rounded, and if the pedal is pushed down VERY gently and slowly, you can get to the floor with the fluid just seeping around the edge of the lip. A harder quicker press will push the lip out and it will seal ok, but it can be scary, and is a sign that it needs work.
jp 83 Euro S AT 49k