Brakes: 140: Odd Brake Problem

[Snowman]

Here's the story, any ideas welcome....

Rallying the 144 yesterday, brake pedal goes soft, warning light on, then pedal hits the floor. Assumed I'd split a pipe but no fluid leak to be seen. Front wheels 'smoking' as we stopped, we'd been working hard.

30 minutes later, I had brakes although mushy and a modest drop in the fluid level and I mean modest. Take out the sensor, pump the brakes, replace sensor and all is well on the warning light front.

The car is running Mintex rally pads and silicone DoT 5 fluid so heat shouldn't be a problem. All flexis are braided and intact. No signs of problems inside the calipers, I've pulled the pads out to check.

My initial thoughts are either the master cylinder seals gave way or I had a pocket of air in the system but to be honest, I'm a bit lost. Today it feels as if the system needs bleeding but I've done two events with no problem since system last evacuated.

Any pearls of wisdom would be appreciated!!

[john h]

Despite the DoT5, it does sound to me like boiled brake fluid.

John

[malb]

One or both front wheels smoking?

My 78 245 did a similar thing at about 50000Km, but with a nice vapour cloud from the front right during braking and warning light.

Girling front caliper has a pair of O rings to seal the passages where the two halves of the caliper mate, and one had been pinched on assembly, and subsequently blew out at max braking on a HOT caliper.

No obvious sign of a leak when investigated five minutes later, but I had seen the vapour cloud and knew where to look.

I know that some 140's in Oz had the same calipers in them, so I wonder if the problem you had might be similar. Would be damn unlucky to have it on both fronts at the same time though, hence the opening question.

[Snowman]

Just the left hand front doing the smoking.

So when it cools, seal remade? This would have been the hottest I've ever got the brakes since the calipers were renewed about four years ago. Pads were also low so I guess plenty of heat transfer to help thing along.

Worth a look, thanks very much.

You're rallying a car with silicone brake fluid? That's nuts. Silicone brake fluid is fine for museum cars, it was originally designed for military vehicles kept in long term storage.

It simply isn't a performance fluid. It sure sounds like boiling fluid, good quality fluid changed regularly is the answer.

[Snowman]

Interesting view bearing in mind it's boiling point. Got into silicone after repeated problems with so-say performance fluid in other cars. Been using it now for five years and this is the first time I've ever had a problem.

Whatever we use, there's always something something better!!

Sure it has a high boiling point, but it more compressable than standard fluid so the pedal is longer.

Secondly if there is inadvertently any water contamination, then it will sink to the lowest point of the braking system, because water and silicone brake fluid are immiscible. With conventional brake fluid the performance degrades, but it is a slow process.

I reckon its a bad choice for any kind of motor sport.

[Snowman]

So having thought some more about this, I'm going to flush the system and go back to a 'normal' fluid. All the seals were new when I put the silicone in, do you think I need to change them again for the new stuff?

[Derek UK]

It sounds as if you managed to get one of those packets of water down near the hot pistons. It then boiled and you lost pressure. If you've been happy with silicone just give it a complete and proper pressure bleed and get back rallying. Castrol LMA a good normal alternative. Changing every 1000 miles when you're competing isn't a bad idea.