Master cylinder seals

[greasynuts]

Hi all,
The motoring season has started - I know this because my wallet is being hammered again!
My question is that I am having to replace the master cylinder and was going to order one from that nice chap at LD parts. However, on looking at the blurb that goes with it, it says that there would be no warranty if I use silicon brake fluid due to potential swelling of the seals.
Has anybody had this problem and is it ok to fit a re-sleeved master cylinder and to use Silicon DOT 5.1???

[martin]

Hiya, why silicone ? I know it doesn't damage paint, and it's not hygroscopic, but it will leave you with a spongy pedal that can't be bled out - and it's expensive. Martin.

[greasynuts]

HI Martin,
Now that seems to be a topic on its own!!
Looking at the various posts and technical stuff, it appears that the Silicone v Mineral debate is 50 -50?
For myself, I have used it for about 2 years and never really seemed to have problems with a soft pedal. I did put the stuff in from when I got the car though so wouldnt really be able to compare the two. Dave.

[73stagman]

This subject comes up time and time again. Those who use it say its great those that don't say don't....

The camp is split so you need to read and read and google and search. You will be non the wiser but you will have made up your own mind by then and be able to join one side or the other of the split camp!

PS.
Im on the use it side (I have used it before in a Dolomite with no problem) and hang the expense!

[greasynuts]

Think you are right with that!!!
Probably stay with Silicone and buy my missus a new tin for her birthday

[4x4]

I believe the problem is that selas that have been exposed to "normal" fluid can then swell enormously when then exposed to silicon stuff. In my work I have encountered numerous instances of different rubber compounds doing strange things (such as swelling hugely) when exposed to different fluids. If you're embarking on a complete rebuild with new seals everywhere, the using silicon is fine. It's primary advantage is that it doesn't absorb water like "normal" brake fluid. Old stuff, as it absorbs water, it's boiling point lowers - so when you're descending mount Everest in your automatic Stag (little engine braking) which you haven't replaced the brake fluid in for ten years, your brake fluid boils and the pedal goes straight to the floor!

[DJT]

Hi all,
The motoring season has started - I know this because my wallet is being hammered again!
My question is that I am having to replace the master cylinder and was going to order one from that nice chap at LD parts. However, on looking at the blurb that goes with it, it says that there would be no warranty if I use silicon brake fluid due to potential swelling of the seals.
Has anybody had this problem and is it ok to fit a re-sleeved master cylinder and to use Silicon DOT 5.1???

DOT 5.1 is polyethylene-glycol based, and is compatible with DOT4. Silicon based fluid is DOT5

Dave

[martin]

I believe the problem is that selas that have been exposed to "normal" fluid can then swell enormously when then exposed to silicon stuff. In my work I have encountered numerous instances of different rubber compounds doing strange things (such as swelling hugely) when exposed to different fluids. If you're embarking on a complete rebuild with new seals everywhere, the using silicon is fine. It's primary advantage is that it doesn't absorb water like "normal" brake fluid. Old stuff, as it absorbs water, it's boiling point lowers - so when you're descending mount Everest in your automatic Stag (little engine braking) which you haven't replaced the brake fluid in for ten years, your brake fluid boils and the pedal goes straight to the floor!

Just a passing thought, hands up those who haven't had to indulge in brake work on a Stag for 10 years ? I feel a poll coming on. Martin.

[4x4]

Off the top of my head I overhauled mine when I first got the car a dozen or so years ago - it needed some new pipework (so I replaced all of it, including braided hoses) and new rear wheel cylinders (both were leaking and had rusty bores). And apart from replacing fluid, that did pretty much see me through for ten years until I replaced the calipers, shoes and drums last summer and have now had to have a major assault on discs, servo and master cylinder.
In 40K miles in a dozen years on a now 40 year old car I don't see that as too much to complain about.
It took me to Monaco and back last summer - only thing that really went wrong was the alternator - a random event no reasonable amount of maintenance can avoid.
Point is though - I know I should replace the brake fluid every few years - not everybody does.
Some cars only get done when some other thing has gone wrong - which in my case has taken over a decade.
Mind you - the bill for the current surgery is going to hit £1500 just for the parts - so averaged over the decade it's not an expense to be ignored......

[73stagman]

The seals in Glycol based fluid and silicone based fluid are always compatible... as far as they are concerned you can mix and match. the problem comes is that the two fluids dont truly mix so if you change you always have some components subjected to old glycol fluid which carries water. the silicone doesnt carry water so you end up with pockets of water sludge silicone and glycol fluid. all in an enclosed system which is then subjected to massive temperature changes within that closed system. You end up with a right mess and no idea of the condition of the metal work in the calipers and master cylinder.
Using silicone brake fluid is no excuse for not servicing the system, in my opinion the only reason that silicone is good to use is that it is easier to handle as any spills just wipe away unlike you rusual brake fluid which takes the paint with it. If you have spent thousands on a paint job then you wont appreciate the little drop of brake fluid that drops onto your front wing whilst topping up or bleeding the system The extra cost of silicone is worht it for that alone. YOU STILL NEED TO REPLACE IT ON A REGULAR BASIS and that doesnt mean once ever decade!

[greasynuts]

All good info this - thanks to all. I think I will stick with DOT 5.1, after all it hasn't ruined my paintwork after dribbling all down the bulkhead.
I replaced most of my seals and the fluid after I bought the car with a 12 month MOT. Turned out 2 of the brake cylinders were seized solid and a bunch of spiders were living in one of the sets of shoes!
Would I need to look at the Servo as well as replacing the master cyl, or is that going to be ok?

[4x4]

Fluid doesn't touch the servo (unless something has gone wrong that is).
The servo simply assists the master cylinder with a pushrod.
If you've been running silicone, you should be fine to keep doing so - it is switching fluid types without changing all the rubberwork that causes problems.

[4x4]

The seals in Glycol based fluid and silicone based fluid are always compatible... as far as they are concerned you can mix and match. the problem comes is that the two fluids dont truly mix so if you change you always have some components subjected to old glycol fluid which carries water. the silicone doesnt carry water so you end up with pockets of water sludge silicone and glycol fluid. all in an enclosed system which is then subjected to massive temperature changes within that closed system. You end up with a right mess and no idea of the condition of the metal work in the calipers and master cylinder.
Using silicone brake fluid is no excuse for not servicing the system, in my opinion the only reason that silicone is good to use is that it is easier to handle as any spills just wipe away unlike you rusual brake fluid which takes the paint with it. If you have spent thousands on a paint job then you wont appreciate the little drop of brake fluid that drops onto your front wing whilst topping up or bleeding the system The extra cost of silicone is worht it for that alone. YOU STILL NEED TO REPLACE IT ON A REGULAR BASIS and that doesnt mean once ever decade!

Good point here - silicone doesn't absorb any water and you run a risk of water pockets - with localised corrosion and boiling possibilities. Even if the fluid was perfectly ok, you would still need to change it regularly to make sure the water was kept out.

[greasynuts]

Thanks to all for this.
I feel happy I'm making the right descision now.

After this 'little repair' I will be embarking on an everything out of the engine bay job.
Engine recon, steering rack, suspension etc. Back end and bodywork alrerady done.
Should keep me busy for an hour or two.