Technical Losing Brake Fluid

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Technical Losing Brake Fluid

OilBurner

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May 18, 2005
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When driving home yesterday in my 1998 Marea TD100 I noticed that the brake pedal started to feel soft. Luckily I was turning into my road at the time anyway so, once parked, I investigated.

With the engine running I pushed the brake pedal down. Initially it was firm but then within a half a second it started to sink to the floor. I repeated this a couple of times with the same result. I then turned the engine off, pumped the pedal a few times to remove any residual vacuum, and tested again. As before, the pedal initially offered resistance but then sank to the floor.

I investigated the brake fluid level and found that the master cylinder was almost empty. However, the warning light hadn't come on! I topped up the resovoir and repeated my tests. As I did so, the fluid level in the resovoir dropped, about 1-2mm for each and every press of the brake pedal.

I jacked the car up and checked all the pipes for fluid leaks, but I can't see any. I also removed all four wheels and checked calipers and pistons for any sign of a leak, but they are all well too. Obviously there are no puddles under the car.

My only thought now is to whether the fluid is somehow leaking inside the vacuum servo unit, because I am struggling to see where else it can have 'disappeared' to as it's certainly not anywhere 'outside' the braking system that I can see.

Before I remove all the pedals and servo: Does anybody have any idea, preferably based on hard-won experience of the same problem, as to where my brake fluid may be going?
 
I went outside to check and then it occured to me: why would there be a brake slave cyclinder on top of the gearbox? Surely all the brake circuits route direct from the master cylinder to the relevant disks/drums? I have no ABS. Are you thinking of clutch, rather than brake?

Having topped it up yesterday, the brake fluid level has dropped again whilst standing overnight. Obviously there is a leak somewhere that doesn't involve pressing the pedal to lose fluid. As my car is clearly unsafe to drive at present, any advice or suggestions would be greatly appreciated.
 
Ah OK, I see what you're saying now, thanks.

However, the position now is that the resovoir has drained itself overnight to the point where there is absolutely no resistance when I press the brake pedal (it just goes straight) to the floor, whereas the clutch still works fine (I can manoeuver around my driveway, slipping it in a controlled manner as required, using the handbrake to stop). This seems to imply that from where the clutch master cylinder draws from the common resovoir onwards is fine (because it has fluid left and still works).

It therefore seems probable that the fault lies somewhere in the braking system. I am suspecting that its around the brake master cylinder area because if I rapidly pump the pedal with the engine off I get a small amount of resistance gradually building up in the pedal (while trapped air is compressed?), before it rapidly dies away again (when the leak takes over?). This suggests to me that there is still fluid in the brake lines themselves, so the leak must be at a higher point in the circuit than that.

As there is still no fluid visible, the only thing I can imagine is that it has leaked to somewhere I can't see. The only place that seems to fit this bill is the large black container (servo unit?) that sits between the pedal and the master cylinder. Is it possible that the master cylinder could suffer a seal failure where the actuating rod enters it, that this could cause a loss of fluid, and that said fluid would then collect inside the servo(?) unit, unseen by me?
 
I first removed the resovoir so that the supply to clutch, front and rear brake circuits was isolated, and then pressed the brake pedal a few times. The effect of this was that the fluid level rapidly went down (1-2mm per pedal press) in the now visible front feed hole, then more slowly in the rear feed hole. Still unable to see any leakage through the brake lines, I surmised that the brake master cylinder must be at fault.

I have now removed the brake master cylinder (and what a horribly fiddly job that is!!!) and, to my horror and disappointment, there is not the slightest hint of a fluid leak on the rear where the actuating rod enters it. It was bone dry. Further, I have bench-tested it and it still successfully forces fluid out of the correct holes, even with a restriction in place in them, with no leakage out of the back. In other words, it seems that the brake master cylinder is working correctly, with no sign of leakage.

So I am now at a complete loss! :confused: I know that I am losing fluid at a horrifying rate, I know that my brakes don't work, and I know that my car is undriveable. However, I have absolutely no idea whatsoever where the fluid is going to :bang:

Does anybody have any ideas?
 
I'm struggling to think how it could, because both the back of the master cylinder (i.e. the bit that pokes 'inside' the car) and the inside of the servo unit (that lives in the driver's footwell) are bone dry. There is not the slightest sign of any leakage whatsoever on that side of the bulkhead, which means that the problem must be 'outside', which is where the four outlet pipes are from the master cylinder are. However, I can't find it anywhere. :cry:

Today's plan therefore is to bleed the brakes again and see if going round that circuit gives me any clues as to where the fluid is going. For this exercise I have disconnected the feed to the gearbox master cylinder and plugged it, so that at least I can exclude something from my tests.

The night before last though, the leak was sufficient that it siphoned (via the master cylinder) the entire contents of the resovoir (i.e. to well below the clutch master take off) to somewhere overnight, so it's a fairly big leak. However no matter how hard I look I cannot see even the slightest trace of any fluid anywhere that it shouldn't be! :bang:
 
The clutch cannot affect this investigation because, as I said earlier: "I have disconnected the feed to the gearbox master cylinder and plugged it."

Also, while the failure of the clutch circuit can potentially lower the level of fluid in the resovoir, (on my car at least) it cannot empty it completely. This is because FIAT have designed the resovoir in such a way that the clutch fluid feed is above the minimum fluid level, so even if the clutch goes open-circuit and takes all the fluid it can, the car will fail-safe with plently of fluid remaining to operate the brakes in perfect safety.

And as I said before, I have lost all brakes. :eek:

I have now traced the fault though :) and it turns out to be a union/hose/pipe above the diesel tank. The reason it took me so long to find this is that the top of the tank is seemingly a curved shape (and no doubt also full of 9 years of road dirt) so has a 'capacity' of more than 0.5 litre of brake fluid before finally starting to drip onto the floor.

My next job therefore is to remove the (2/3 full!!!) 67 litre diesel tank from the car so I can establish exactly which union/hose/pipe has failed, and of course gain access to replace it. I can't honestly say that I am looking forward to this, but it must be done. :(
 
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