Speed-Talk is running on www.Speed-Talk.com

IMPORTANT: Update your bookmarks to https://www.speed-talk.com/forum/
(Right-click the URL and select "Bookmark this link")

Non-vented brake MC reservoir?

Shocks, Springs, Brakes, Frame, Body Work, etc

Moderator: Team

Post Reply
turbo camino
Pro
Pro
Posts: 423
Joined: Sun Aug 10, 2008 11:51 am
Location:

Non-vented brake MC reservoir?

Post by turbo camino »

Mid-'90s Honda Civic, complaint of long brake pedal. I check the fluid level, hey that's strange, it looks completely full all the way to the top. Crack the cap loose and -FLURP- fluid level drops down to the full mark where it should have been all along.

I am assuming that over time and/or with the colder weather, the airspace above the fluid level gradually got burped out until there was no air left, and the vacuum that built up was keeping the caliper pistons retracted.

I have inspected the cap and cannot find a vent hole or slot or anything, it looks to be completely sealed. This is an all-plastic cap with an oring, no rubber isolator diaphragm like is typical on everything else I'm familiar with. How the hell are the brakes supposed to function if it's allowed to build a vacuum above the fluid in the reservoir?! What am I missing here?
DON'T PANIC
lefty o
Guru
Guru
Posts: 3088
Joined: Tue Oct 15, 2013 8:50 am
Location:

Re: Non-vented brake MC reservoir?

Post by lefty o »

i dont know what you are missing, but if it has been that long its probably time to flush the whole system and bleed it.
turbo camino
Pro
Pro
Posts: 423
Joined: Sun Aug 10, 2008 11:51 am
Location:

Re: Non-vented brake MC reservoir?

Post by turbo camino »

Master cylinder was replaced 3 years ago because of bypassing with light pedal when cold. System was flushed at that time. Pads were replaced 2 years ago, no problems since, so I assume the cap hasn't been opened since then. How often should the system be flushed/serviced to avoid this? Every 6 months?
DON'T PANIC
turbo camino
Pro
Pro
Posts: 423
Joined: Sun Aug 10, 2008 11:51 am
Location:

Re: Non-vented brake MC reservoir?

Post by turbo camino »

Fluid in the reservoir drops as pads/shoes wear. Something must be allowed into the reservoir to replace the volume of fluid that gets relocated into the calipers. On a MC with a rubber diaphragm sealing the reservoir, the rubber allows the pressure to stay equalized but still isolate the fluid from moisture in the atmosphere. This cap has no way to equalize the pressure back to ambient as the fluid level drops in normal use. The brake system cannot function if there is a vacuum in the reservoir. Hence, I must be missing something, this cannot be normal operation. What is it? The answer cannot simply be 'service it more often.'
DON'T PANIC
David Redszus
Guru
Guru
Posts: 8094
Joined: Tue Nov 27, 2007 9:27 am
Location: Chicago
Contact:

Re: Non-vented brake MC reservoir?

Post by David Redszus »

turbo camino wrote: Fri Nov 20, 2020 11:55 am Fluid in the reservoir drops as pads/shoes wear. Something must be allowed into the reservoir to replace the volume of fluid that gets relocated into the calipers. On a MC with a rubber diaphragm sealing the reservoir, the rubber allows the pressure to stay equalized but still isolate the fluid from moisture in the atmosphere. This cap has no way to equalize the pressure back to ambient as the fluid level drops in normal use. The brake system cannot function if there is a vacuum in the reservoir. Hence, I must be missing something, this cannot be normal operation. What is it? The answer cannot simply be 'service it more often.'
You might check the shop manual regarding fluid circuit diagram and proper bleeding procedures.
Once owned a Civic that had a pressure equalization valve installed which mechanics always seemed
to miss. No one knew what the damned thing actually did.
turbo camino
Pro
Pro
Posts: 423
Joined: Sun Aug 10, 2008 11:51 am
Location:

Re: Non-vented brake MC reservoir?

Post by turbo camino »

Unless there's an equalization valve located in the side wall of the reservoir, I don't see how it could possibly affect this situation. This is a vacuum being created in the reservoir, between the top of the fluid and the underside of the cap. Something has to be able to get in there to replace the volume of fluid that exits out to the brakes.
DON'T PANIC
turbo camino
Pro
Pro
Posts: 423
Joined: Sun Aug 10, 2008 11:51 am
Location:

Re: Non-vented brake MC reservoir?

Post by turbo camino »

I disassembled the lid and found the vent hole!
top.jpg
And now I'm more confused than before. Vent was not blocked, everything was as clean as you see it there, there are standoff ribs on the top surface that prevent the outer cover from accidentally sealing against the inner part in the photo, etc. But there was very definitely a vacuum in the reservoir before the cap was removed. How?
You do not have the required permissions to view the files attached to this post.
DON'T PANIC
Post Reply