Brake fluid overflowing - WTF

[roseytvr]

I have been on a track day today and have had a problem with brake fluid leaking past the diaphragm in the master cylinder cap and stripping all my nice new paint on the inner wing. I have a new servo, master cylinder, DOT 5.1 fluid, braided hoses, 4 pot callipers and DS2500 pads and they have performed perfectly all day.

When I rested up during the day I noticed the leaking fluid coming out of the top of the master cylinder. As I understand it brake fluid will only expand if its exposed to moisture, which it wasn’t. I have got back home and the fluid level is now quite low because of the amount of fluid lost. A couple of questions to those in the know:

1. Should the fluid come past the diaphragm? I guess the alternative would mean once the relief space inside the master cylinder reservoir was full the brakes would lock up so I presume it should come out to avoid this.
2. Why would the fluid level rise in the first place and then fall later?

Any ideas guys?

[Toltec]

Thermal expansion of the fluid I suspect.

[QBee]

Observation - I have Dot 5.1 fluid, reservoir was full when I checked it on Thursday ready for the TD, no problems with expansion, but I will check this morning just in case there has been a spill. I was well on it all day, with big front brakes, so heat could have been an issue. No issues on previous TDs, using standard TVR setup with DS2500s.

Just a thought - I assume it was coming out of the top of where you fill it up. Was the cap on tight and is it sealing properly? Mine is quite a fiddle to get fully tight.

[my250gt]

exact same thing happened to me on a bedford trackday, makes quite a mess on the inner wing doesnt it! I was advised to put racing fluid in next time but havent got around to it yet so cant say if it solved the problem or not, but my guess it would. I blamed my driving style, on the brakes too much !! ;-)

[Graham]

Most likely just fluid sloshing around while on track and leaking through breather hole and or cap. Plus a bit of thermal expansion .

[roseytvr]

my250gt said:

exact same thing happened to me on a bedford trackday, makes quite a mess on the inner wing doesnt it!

It certainly does and I'm gutted as I had recently resprayed the inner wings. I will now cover it with some stick on covering of some sort

Graham said:

Most likely just fluid sloshing around while on track and leaking through breather hole and or cap. Plus a bit of thermal expansion .

Unfortunately there was certainly some form of expansion going on as the fluid completely filled the reservoir and you could see through the top of the reservoir the relief space had disappeared.

However I thought the whole idea of brake fluid was for it to resist thermal expansion. I have lost at least a cap full of fluid out of the top and at least another capful to fill the air space in the reservoir. So it has either expanded or reverse pressure built up!

[phillpot]

If discs are getting very hot could they be expanding, in effect getting thicker, so pushing pistons back in calipers?


Just an idea, don't shoot if its a stupid one!

[Steve_D]

If the wheel bearings have an amount of play then in hard cornering the disc can push the pistons back.
If a racing bike gets a bit of a 'tank slap' the rider will alway give the brakes a dab before the next corner to re-set the pistons.

Steve

[Graham]

Steve_D said:

If the wheel bearings have an amount of play then in hard cornering the disc can push the pistons back.
If a racing bike gets a bit of a 'tank slap' the rider will alway give the brakes a dab before the next corner to re-set the pistons.

Steve

If you were getting pad knock back, youd notice it with a long pedal, before it firmed up as normal

[roseytvr]

phillpot said:

If discs are getting very hot could they be expanding, in effect getting thicker, so pushing pistons back in calipers?


Just an idea, don't shoot if its a stupid one!

Not one for shooting as any suggestions are welcome, I am not sure they would thicken up by the amount of fluid lost but if the same happened both sides who knows!

Steve_D said:

If the wheel bearings have an amount of play then in hard cornering the disc can push the pistons back.
If a racing bike gets a bit of a 'tank slap' the rider will alway give the brakes a dab before the next corner to re-set the pistons.

Steve

This could be worth investigation as the steering certainly felt heavy when cornering hard, I had problems with heavy steering and lack of confidence all day. Certainly something felt not right but I cant work it out yet. I have changed all the steering components in the last 18 months except the wheel bearings and UJs. To be added to winters list!!

[QBee]

roseytvr said:

This could be worth investigation as the steering certainly felt heavy when cornering hard, I had problems with heavy steering and lack of confidence all day. Certainly something felt not right but I cant work it out yet. I have changed all the steering components in the last 18 months except the wheel bearings and UJs. To be added to winters list!!

Not saying he is wrong, it sounds like it might be the solution, but mine was heavier when cornering hard too. However It really inspires confidence, as i can hold onto it (like my life depended on it on some corners!!) and feel exactly where i am in the corner in terms of what both ends of the car are doing.

Note for next time - I am now wishing we had spent more time talking to each other, as a group, and time driving each other's cars. We just might have sorted out a few of each other's problems.