Master Cylinder

[Kev1990]

Hello I am replacing my master cylinder this Friday just wondering how much brake fluid do you normally use when bench bleeding master cylinder and can I bench bleed it while it's on the car? Also what size are the nuts to the bracket that plate on the back part of it any help or tips is greatly appreciated thank you in advanced.

[wildBill83]

Just buy a large bottle of fluid, use it until all air is purged. The bolts should be 10MM.

Do not use old fluid and throw your fluid away after 30 days of opening the bottle.

I haad no problem bleeding my master through the slave via a vacuum bleeder.

[johnl]

Do not use old fluid and throw your fluid away after 30 days of opening the bottle.

I suspect this is a thing put around by fluid manufacturers to sell more fluid. When the cap is refitted to the bottle then it should seal perfectly well, so no moisture should be able to migrate past the cap anymore than it could migrate through the sides of the bottle.

Sure, if the cap is left off for a significant period of time or not properly tightened then there is likely to be an issue, but the bottle is at least as well sealed as the reservoir on the braking system, and probably better. In the old days fluid reservoirs were often vented to atmosphere (with a tiny hole in the cap) to allow for pressure changes in the reservoir, and this led to fluid contamination with water over time, necessitating the need to change fluid regularly.

Modern reservoirs aren't vented in a manner that easily allows atmospheric air to contact the fluid, but over time some small amount of atmospheric air can pass into the reservoir as a result of pressure changes as fluid moves out of and into the reservoir every time the brakes are used. This lesser exchange of internal / external air from and into the reservoir reduces the need to change fluid as often as older style systems. This pressure variation (each time the brakes are used) doesn't happen in the bottle, only what could occur due to atmospheric pressure changes, which the cap should cope with easily.

To check bottle cap tightness I invert the bottle and give it a good hard squeeze, if it's properly tight and sealed then no fluid will leak out.

[wildBill83]

While your logic is very clear and concise, I want to point out two things:
1. Water tight is not the same as air tight.
2. The bottle comes air sealed from the factory and is bottled with nitrogen to prevent oxidation and moisture ingress. If you open the bottle you expose it to the air, when you use 3/4 of the bottle doing your fluid change then bottle it back up trapping a good deal of air in the bottle you will reduce the shelf life of the fluids.

There are studies on shelf life of opened dot3 fluids out there somewhere.

[johnl]

Perhaps.

I've been using fluid from containers that have been opened an indeterminately (but definitely long) time ago for ages (like forever...). I've never had an apparent issue, whether on my road cars or with my racing karts, i.e. I've never boiled the fluid nor had internal corrosion problems (i.e. corrosion that couldn't be attributed to fluid that hadn't been changed regularly under prior car ownership). I change the brake fluid about once a year.

Even if there is (and I'm not saying there is none at all) some theoretical affect on the fluid (of the container having been opened more than "30 days" ago), I'd be amazed if new fluid from a bottle (that has previously been opened some long time ago) and properly resealed was not in significantly better condition than the fluid in the braking system, even if the fluid in the system were to have been changed only a relatively short time ago...