Leaking brake reservoir cap.

[Speedtrip]

I've previously queried a leaking reservoir cap under warranty. Other than trying to tighten it more I don't think the dealer did anything much and fobbed me off.

Today I've noticed it's still leaking, assuming this isn't unusual what's the normal fix?

[ECR]

Fit the ap master cylinder race cap to stop the leaks

[Jim 123]

There can be a tendency to put too much more fluid in the reservoir than needed on initial fill, and the standard seal is not great.  A syringe can be used to lower the level to the marker line. Make sure any earlier spill from the cap is dried off and removed as it does tend to hang around the big plastic warning label under the cap, which can convince you that the leak persists. 

[Speedtrip]

Thanks Jim,

The caps definitely still leaking as there was a deposit below, not just a dribble or smear. 

Thanks also ECR,

The car came with what may be the cap you're referring to. It looks like a cap with a black bellow? If correct this would do away with the level sensor... In fact this:

https://caterhamparts.co.uk/master-cylinder/914-cap-race-master-cylinder.html

[ECR]

Yes, the race cap has a bellows. Be careful not to overtighten It when fitting. A smear of brake fluid on the sealing face helps

this does do away with the warning light though .....

[Speedtrip]

Thanks ECR.

Doesn't seem right the standard cap is allowed to leak. Is it "enthusiastic" driving which causes the problem.....so my fault then?

Not sure I'm to keen on loosing the warning light.

[The_ASH]

I notice this happens in the race motorbike world, one workaround seems to involve a wrist sweatband (like that used for tennis) or similar fitted around the cap so anything that comes out is absorbed and doesn’t spread.

[ECR]

The standard cap has to have a breather hole somewhere to accommodate expansion of the brake fluid in the system. The race cap bellows is there to compress and do the same job

[Speedtrip]

Thanks again,

Is a functioning brake fluid level warning light an MOT requirement?

The idea of running without a light to stop fluid escaping all over the inside of the car doesn't sit well with me. I could have dire consequences!

[Jonathan Kay]

Is a functioning brake fluid level warning light an MOT requirement?

Not explicitly. but if the system is there and not working it could fail under some catch-all wording. I haven't heard of any problems in practice.

New Inspection Manual, see Section 1.

Jonathan

[MADMALC]

Hi S,

Unfortunately it is a case of TADS. I found that lowering the level as much as possible helped then a wipe round after every spirited run. Just something you learn to live with. A 'race' cap for track and competition use may suit you, you would normally check fluids before every run so little value in the low level light in those situations.

 

[Speedtrip]

Thanks Jonathan.

To me that's a fail. If what I'm experiencing is normal then it sounds like CC's strikes again, poorly developed product but this time with safety implications!

Brake fluid really shouldn't need topping up on a regular basis under normal usage.

[Jonathan Kay]

To me that's a fail.

More, please...

Jonathan

[Speedtrip]

Jonathan,

I'm far from an expert but I'd say the warning light is a safety feature intended to alert the driver to reduced fluid levels from a failure/leak. If the floats not present when CC's "solution" is implemented then it can't do its job....simples.

The manual may not specifically cover it but as all cars are now fitted with a warning its presence must be required, even if it's only in the spirit of the MOT process. It pretty basic really.

[Jonathan Kay]

Thanks

Jonathan

[bjw]

A brake fluid level indicator problem is rated only at minor in the new scheme of things, not on its own a failure:-

Section 1.1.10.
defect (f): Brake fluid warning light illuminated or defective  - Minor
defect (g): Incorrect functioning of brake fluid level warning device  - Minor

Cheers

.

[MADMALC]

Don't keep topping it up, if the warning light is off there is enough fluid in the system. If you keep topping it up it will leak it out again. If you don't it will find a level where it is able to cope, hopefully at or above the 'minimum' mark.

[Robs7]

I had the same leaking problem and couldn't get a replacement from any car spares. Went to a plumbers merchant and bought a similar one for 10p. Had to slightly trim the inner side of gasket to fit but job done. No more leaks

[SimonR300]

Just to add to ECR's advice;

My car came with a bad leak from the cap, so bad that the ECU below the master cylinder was sitting in a puddle which had burnt the paint across the deck.

I tried new caps with a fresh rubber seal but it still leaked.  Then I realised that the bellows was full of fluid which could obviously escape through the breather/pin hole in the cap.  So I bought a few new bellows, only to find that they also where not air tight. 

As I understand it, the bellows should create a dry air seal/chamber between the breather hole and the fluid so as the fluid level goes up and down, the bellows can go up and down, drawing air in and out of the cap.  The bellows therefore stop moisture rich air being hydroscopically drawn into the brake fluid.  The bellows should be fitted to the cap first or there is a chance that the cap will not make the correct seal with the bellows and rim of the reservoir.  It also should be compressed if you can or it will displace quite a bit of fluid and not function correctly.

Standard race car practice seems to be to add a piece of paper towel/cloth over the cap with a cable tie to absorb any other leaks.

I also found that previous race mechanics had over-tightened the cap in desperation and damaged the thread on the reservoir, so there was a leak from the cap thread as well.  Although difficult to obtain, I fitted a new reservoir and finally have a leak free set-up.

[Rich Goddard]

Just sharing a bit of new learning (for me), for the hopeful benefit of others.  Realise this might be common knowledge for everyone else who uses the AP race cap for their master cylinder....but wasn't obvious to me.

Replacing my standard cap with race cap I stumbled across the info below (from AP website) which says you must first remove the plastic baffle and rubber washer inside the cap before fitting the bellows.

The bellows then takes a little bit of stretching and persuading/poking to seat properly around the plastic 'upstand' ring within the cap.  But when it does it feels very well seated.

I wonder if this might be one reason why some suffer leaks from the race cap (ie the bellows fails to seat/seal properly against the baffle and rubber washer.

 

[7 wonders of the world]

Girling cap is the same thread and can be used with the there was also an issue with some of the caps having poorly made threads which would not fully tighten too, for  reference the Girling cap can be used with the AP bladder.

[StevehS3]

My 310R leaky cap story. The original with level switch leaked. Replaced the cap with another, still leaked. Removed some fluid so it was just above the minimum and the dash light occasionally flashing, stopped the leak but not totally happy to take it on track with the light flashing under acceleration. 
Took 7WotW's advice and put a smear of RRG on the rubber seal, filled fluid to normal level and bingo, no leaks even on track. I think the RRG creates a better seal but also stops the rubber seal creasing as the cap tightens down on to it.

[Nigel B]

I simply added an additional rubber seal underneath the standard AP cap. 

I felt the cap was 'bottoming out' before the rubber seal was fully compressed, as there was little sense of 'squish' as the cap tightened up, it seemed to simply come to an abrupt halt at the bottom of the thread. With an additional seal underneath the cap, there was a definite sense of the rubber compressing as the cap tightened down.

It did the trick. That was three & a half years ago & I've had no further leaks.