While we're on the subject of master cylinders...

[Mark Worthington]

I have what may be a stupid question about the brake master cylinder on my RF. The top of the MC has two little rubber accordian boots, presumably which expand under vacuum and reduce the amount of fluid in the MC exposed to air. I managed to spill brake fluid all over the place last night while pressurizing the system when I put the cap back on the MC. Am I supposed to deflate those boots somehow before I put the top back on the MC? One of my boots has expanded and the other hasn't. I just want to make sure I'm setting this up the right way.

[Hershal Byrd]

Mark,
Those boots are there so when the pads wear the fluid takes up the slack. The boots expand to displace air and keep the pedal pressure the same by preventing air from entering the system. If you were to replace the worn pads on a brake system you would have to squeeze the wheel cylinders back in to fit the new pads. When you do this all the fluid then fills the master cylinder back to its original level. This is when you would need to deflate both the little boots and replace the cap.

Hersh [img]/ubbthreads/images/graemlins/smile.gif[/img]

[Mark Worthington]

Thanks, Hersh. So this means I should deflate them on initial filling of the master cylinder, right?

[Robert Logan]

Mark,

CORRECT.

Hershal - an excellent explination.

Best wishes,

Robert

[ChrisL]

[ QUOTE ]
The boots expand to displace air and keep the pedal pressure the same by preventing air from entering the system

[/ QUOTE ]

Although the principle is the same, I have a different understanding of the reason for the boot. Is it not there to create a barrier between the external air and the air chamber for the purpose of minimising moisture exchange from air to brake-fluid, and hence extending the effective life of the fluid?

I'm happy to be corrected.

[patbarry]

Your right, because the boot has no ability to resist pedal pressure. All the pressue comes inside the chamber of the cylinder, not the reservior. If the reservior was pressurized, we would be blowing the tops off these plastic things. The boot is there to keep trash and moisture out of the fluid.

Or at least that is what I was taught many years ago.

[Hershal Byrd]

Guys,
I think we are both right. I just didn't mention the whole story in order to keep it simple. So here's rest of it...
The cap on the brake fluid reservoir has a hole for air, or is vented, to allow the fluid to expand and contract without creating a vacuum or causing pressure. A rubber diaphragm goes up and down with the fluid level's pressure, and keeps out any dust or moisture.
If you take out the rubber diaphragm then you would have to constantly check and add fluid as the brake pads wear. If the fluid gets too low then air would be introduced to the system causings a spongy pedal when they are applied.
Remember the old fruit jar master cylinders used in the fifties and sixties. They had a single resevoir and no diaphragm expansion seal.
I did not mean to imply that there was pressure in the resivoir. As Pat says it would blow the cap clean off. [img]/ubbthreads/images/graemlins/grin.gif[/img]

Hersh [img]/ubbthreads/images/graemlins/smile.gif[/img]

[Mark Worthington]

I've read that the OEMs go to great lengths to keep their brake fluid isolated from air, to minimize potential for water to get in the fluid. I felt hurried the other night keeping the top off for a minute while I added fluid (it was pretty humid waether).

[ChrisL]

So if I understand the air-in-system problem corretly, the expanding diaphram displaces the missing fluid so that the effect of surge can be minimised?

That would make sense. The old fruit-jar reservoirs minimized the surge effect by being round. Movement of fluid in these old units would be directed straight back to its origin because of the round shape of the vessel. So I guess, in a dual cylinder system, the surge problem would be greatly enhanced under braking or acceleration because of the irregular shape of the reservoirs, hence the greater need for the expanding diaphram.

If I’ve understood that correctly I will be able to sleep nights again [img]/ubbthreads/images/graemlins/tongue.gif[/img]

[Trevor Booth]

1) reduce surface area exposed to the atmosphere and intended to minimise moisture absorption. primarily inteneded for those who do not change the fluid on a regular basis and or use crap fluid.
A tall small diameter circular vessel will have a smaller surface area (for equal volume)
2) prevent spillage under surge conditions,
incidentally a circular vessel concentrates surge at the tangential point of the vessel whereas a rectangular vessel does not, but it does have more energy. Try it with a cup of water and a rectangular or square cake tin (or similar)

[Hershal Byrd]

I think you gentleman have it!
I'm no scientist but Trevor's explanation sound good to me. [img]/ubbthreads/images/graemlins/grin.gif[/img] Where's that Ron Earp when you need him.. [img]/ubbthreads/images/graemlins/wink.gif[/img] [img]/ubbthreads/images/graemlins/grin.gif[/img]

Hersh [img]/ubbthreads/images/graemlins/smile.gif[/img]