are brake fluid reservoir caps sealed?

Joined
Jan 17, 2003
Messages
3,463
Location
Coastal South Carolina
the reservoir like on 2000-2010 toyota camrys, the cap is a rubber part that snaps over the top of the reservoir- do they breathe? or as the fluid level drops due to pad wear does a vacuum develop in the reservoir?
 
Joined
Jun 15, 2003
Messages
36,782
Location
ME
There's no vacuum. A tiny hole somewhere. If you flex the cover when it's on, you'll hear it.
 
Joined
Dec 19, 2004
Messages
7,430
Location
beaver land EH?
most imports, incl. Japanese automobiles, (camry incl.) has a slit on the rubber seal diaphram underneath the reservoir cap. It's meant to allow atmospheric pressure balancing meaning that air (incl moisture) can get in as the fluid level descends... Q.
 
Joined
Jun 28, 2003
Messages
6,423
Location
Illinois
It has to be vented otherwise as the pads wear and the caliper piston extends it would be pulling a vacuum on the reservoir as the fluid level drops. Is there a whoosh from air going into the MC when you take off the cap?
 
Joined
Dec 5, 2009
Messages
28,122
Location
Regina, Saskatchewan, Canada
They're sealed, but not at pressurized, to make it simple. The rubber diaphragm allows the fluid level to drop without bringing in moist, outside air into contact with the fluid.
 
Joined
Jun 28, 2003
Messages
6,423
Location
Illinois
Originally Posted By: Garak
They're sealed, but not at pressurized, to make it simple. The rubber diaphragm allows the fluid level to drop without bringing in moist, outside air into contact with the fluid.
So how does the air get in there when the fluid drops an inch in the MC reservoir?
 
Joined
Dec 5, 2009
Messages
28,122
Location
Regina, Saskatchewan, Canada
The diaphragm (assuming no leaks) moves closer to the fluid, and the air that gets in is actually separated from the brake fluid by the diaphragm. It's between the lid itself and the diaphragm. On the old Chev taxis, it was quite noticeable over the life of a set of pads. With new pads, the master cylinder would be full, and you'd make sure the diaphragm was seated correctly and not all crumpled or anything. As the pads wore and the brake fluid went down, the diaphragm would be distended "down" towards the brake fluid. Replace the pads and straighten out the diaphragm and the process starts all over.
 
Joined
Oct 24, 2008
Messages
1,161
Location
New Zealand
I think sealed master cylinder reservoirs (with a flexible bellows) used to be more common (GM?) but I haven't seen one in years. The fluid level cycles a tiny amount on every brake application so a vented cap will draw in moisture relatively quickly.
 
Joined
Dec 5, 2009
Messages
28,122
Location
Regina, Saskatchewan, Canada
Originally Posted By: Kiwi_ME
I think sealed master cylinder reservoirs (with a flexible bellows) used to be more common (GM?) but I haven't seen one in years.
I wouldn't be surprised. My old truck, of course, still has such a system. Having done brakes a bajillion times on the taxis (mostly the same make and model) has made me relatively familiar with one setup. Ask me what the master cylinder cap looks like on something like a modern Focus or a Civic and I'd be lost. wink
 
Joined
Jun 28, 2003
Messages
6,423
Location
Illinois
Originally Posted By: Garak
The diaphragm (assuming no leaks) moves closer to the fluid, and the air that gets in is actually separated from the brake fluid by the diaphragm. It's between the lid itself and the diaphragm. On the old Chev taxis, it was quite noticeable over the life of a set of pads. With new pads, the master cylinder would be full, and you'd make sure the diaphragm was seated correctly and not all crumpled or anything. As the pads wore and the brake fluid went down, the diaphragm would be distended "down" towards the brake fluid. Replace the pads and straighten out the diaphragm and the process starts all over.
If an inch of fluid drops in the MC that's a lot of vacuum to hold back. The flexible bellows would be sucked down and be distended for sure. And there would be quite an inrush of air when you took the cap off.
 
Joined
Dec 5, 2009
Messages
28,122
Location
Regina, Saskatchewan, Canada
Actually, no, there wasn't much of a fight with that style cap in my experience. Above the diaphragm was room for air to fill, so that side didn't face any vacuum. However, if there was a brake fluid leak, the diaphragm would be very distended and it could be a bit of a chore to get it sitting nicely again. Now I'm all curious and will have to take a look at the G37's reservoir and see how it compares to the older designs. wink
 
Joined
Oct 24, 2008
Messages
1,161
Location
New Zealand
It must be vented somewhere, look for a microscopic cut in the rubber. The fluid in the MC must be kept near to atmospheric pressure for everything to work safely and no manufacturer is going to take chances with brake systems. On my car (Suzuki) I've sealed the cap and installed a nipple so I can attach an air pump for pressure bleeding. But rather than leave it open in daily use (as I should,) in BITOG fashion I've attached an empty toothpaste tube to act as a bladder. The interesting thing is that rather than collapse over time with brake wear it pumps up tight after just a few days.
 
Joined
Jun 28, 2003
Messages
6,423
Location
Illinois
There's also the heating and cooling the fluid goes through in the MC due to under hood heat.
 
Joined
Aug 20, 2007
Messages
1,841
Location
.
Brake fluid of has the ability to absorb water (hydroscopic), so the brake fluid reservoir must be sealed properly. It is a DOT reg. This even applies to four wheelers and motorcycles. If you have a vented system, it is not DOT compliant. I tried to search these words but came up with nothing. I remember reading such verbiage on car owners manuals from the 50's and 60's when you could simply remove the rubber seal from the cap. You will see a hole in the cap on many master cylinders, but it is above the reservoir diaphragm.
 
Top