Master Cylinder Caps

[spyryder]

and in the closed position....the slit is barely visible. :doorag:

 

[Way2Fast]

What you may be missing is that the cups (the rubber seal between the caliper bore and piston) are specifically designed to seal against pressure from one side only. They are tapered in towards the fluid side. When pressure is applied it actually works to press the rubber seal against the piston (or in some cases the caliper bore). The harder you press on the brakes the harder the fluid presses against the rubber seal and the tighter the seal is. Very effective.

However, it works in the opposite way from the reverse side (the air side in this case). If you looked at the seal you would see what I mean. I could not find a good picture of what I'm trying to describe so I made one. Pretty ugly I'll admit. But if you can get by that I hope it demonstrates what I'm trying to describe. The seal can go on either the piston or the bore. This picture is a piston mounted seal but it's the same thing either way.

The seal goes all the way around the piston, of course. But for demonstration purposes I just show the seal as 2 tapered wings (plus the fact that I can't draw well enough to do more than this anyway). The clearance to the caliper bore is much less than this and distorts the seal somewhat.

As you can see, when fluid pressure is applied it pushes against the rubber expanding it against the bore giving a great seal. The opposite effect is true when pressure is applied from the opposite side. Any negative pressure inside the caliper will try to equalize by pulling air past the seal. It takes a relatively small amount of pressure differential to allow air to sneek past the seal. The seal simply is not designed to seal against much pressure from the air side. Nor does it need to in a properly working system.

The other problem here is that soon, Lamontster is going to tell me to take this to the Tech forum! :joke:

Your hand drawn diagram is a lot better than I could do and it answers the question I had. I never bothered to analyze the type seal used for the pistons.

Thanks,

Richard

 

[Way2Fast]

Here's one site I found:

Title:Assured venting master cylinder diaphragm apparatus and method Document Type and Number:United States Patent 4987740 Link to this page:http://www.freepatentsonline.com/4987740.html Abstract:A diaphragm for a vented master cylinder reservoir which further includes means for opening the slit thereby assuring venting of the master cylinder reservoir is provided. The slit opening means are configured in such a manner that contact of the slit opening means with the master cylinder cap will cause the diaphragm to be stressed and to elastically deform, therefore assuring the opening of the venting slit.

As far as my Chrysler, I'm the original owner and the only person who's done any service on it...that's why I thought I'd mention it. I can assure you that everything is as it should be. Also, the cap looks identical to the Spyder's caps....but there is only one cap and a fairly large reservoir. That diaphragm would be way too small to compensate for the amount of fluid in there. Also, older cars were all vented directly to the atmosphere anyways, so I can't really see a big problem with moisture.

Now the Spyder...perhaps all of them were supposed to have that V shaped slit in the first place?.....Why are some of them on the recall list and not others? I'd like to see a cap from one that isn't on the list and compare the two....see what the difference is. :dontknow:

I have never seen any car, regardless of age (going back to the 50's anyway) that does not have an expandable diaphragm in the master cylinder cover.

Moisture can be a BIG problem with hydraulics. It can corrode the caliper pistons and also cause the fluid to boil during heavy breaking.

I understand that the slit cut in the rubber diaphragm is in the closed position when not under the stress of trying to correct the effects of a vacuum. But even if it only opens momentarily it still allows air to get past the diaphragm and with it accompanying moisture. Perhaps the air/moisture entering through the cap is preferable to the possibility of air being sucked in through the caliper pistons under a high vacuum load.....and it should be. But in a well designed system I still contend that NO outside air should be able to make contact with the hydraulic fluid in any way, manner or form.

Richard

 

[Way2Fast]

When I started this thread my question was ...had anyone besides Lamonster bothered to remove the caps on the master cylinder to check if the rubber diaphragms were pulled down ? His were pulled down, which could be because of brake pad wear. One of mine was also partly down with less than 2 miles on the vehicle. I attributed this to the change in temperature between Canada and Florida. Come on guys, unscrew the caps and post what you find.

Richard

 

[skaha]

Both caps on #195 have a groove milled in them from the bottom to the middle of the cap inside. Just like the picture that Spyryder has posted. ( the cap with the yellow plastic) My machine is not on recall list.

 

[spyryder]

Both caps on #195 have a groove milled in them from the bottom to the middle of the cap inside. Just like the picture that Spyryder has posted. ( the cap with the yellow plastic) My machine is not on recall list.

Thanks Skaha.....does the diaphragm also have a slit in it? You'll probably have to wipe it off as it's hard to see.

Arnie

 

[csmead]

I have been following all threads on and about the brakes.

Has anyone been able to confirm that this braking issue is the reason that there has been a "Stop Issue" order on all Spyders. My dealer said that even if my bike came in, he could not give it to me!

 

[skaha]

Both caps on Spyder 195 have a small ( V ) sliced in the rubber along with a groove that has been milled into the cap on the inside from inside rim to the center of the cap.