Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Brake fluid reservoir SUPPOSED to be open to air?

Car: 1974 MGB

I'm just about finished my brake m/c rebuild. I get around to cleaning the cap of the reservior, and I notice that it's actually two pieces. I pop them apart and clean them, and I realize that there is a hole in each one of them.

So, the end result is that the fluid in brake master cylinder is open to the air. I can only assume that this is by design, since the holes are quite obviously intentional.

I don't recall any of the reservior caps I've ever dealt with on modern cars being vented. Especially since brake fluid is hygroscopic...

Didn't find anything in the archives on this topic, except a few mentions of the vented cap.


Why is it vented?

Anders Green


It is vented so that air can enter as the system as the pads / shoes wear so that the overall system volume increases.


All brake master cylinders, modern or antique are vented to avoid drawing a vacuum as wear on the brake shoes/pads occur, causing fluid to be drawn out of the M/C. Some modern cars use a stretchable membrane to accomplish this venting without air actually entering the system. Cheers - Dave
David DuBois

It is there for a good purpose! In the past I have but a piece of tape over it while working on broken brakes lines. The tape keeps the fluid in the system while a brake hose is disconnected for a repair. I then only have to do minimal bleeding when I re-connect them.

After learning this little tidbit, it is obvious to me that blocking the hole is not a good idea.

By the way, if you want a really firm pedal after changing brake fluid, follow what I call the "R. Smith painless brake fluid change method". Do the following:

1. Gather all the things you need - empty brake fluid can, tubing, new brake fluid, etc.

2. Go to the grocery and buy a six-pack of your favorite beer.

3. Start with the wheel farthest away from the master cylinder, hook up the tube and put it in the empty brake fluid can.

4. Remove the master cylinder cap.

5. Open the brake bleeder.

6. Open a bottle of beer and enjoy it while you watch the fluid level in the reservoir very slowly fall.

7. Top up the reservoir whenever the fluid gets low.

7-1/2. Open new bottles of beer as needed.

8. When you see that the fluid coming through the tube is clean and new, close the bleeder screw.

9. Repeat the process for each wheel, always doing the next farthest away wheel from the master cylinder.

Takes about 4 hours and a six-pack of beer. Great for a Saturday afternoon and gives you an incredibly firm pedal.
Richard Smith

It also allows for expansion of the fluid as the system heats up under heavy braking. Without the breather it would apply the brakes as the fluid expands! Personally I like to bleed mine in much less time than an afternoon, and without the beer so I can drive it afterwards :o)

But sealing it off *is* a good way of limiting leakage while you have any hydraulic connections open.
Paul Hunt

Richard.....and a headache!!!

Okay Richard- How many times did you screw up and pour the beer in the Master Cylinder. Only brake system I know with a head
greg fast

Never have I screwed up and poured the beer in the master cylinder...

My method does, however, work well! One of the main benefits is the slowness with which it occurs - I think the slowness better allows the air bubbles to move along with the fluid completely through the sytem and exit into the bottle.

And Paul makes a good point that I have observed over the years. Someone pushing very hard on the pedal and then quickly opening and closing the bleeder is the most effective way. Forget the rapid pumping. It is definitely easier to bleed the clutch this way.
Richard Smith

Richard -

I've also used this method, beer included. :-) It takes patience but I found, as you did, that it did a better job of bleeding than the open valve / push pedal / close valve / release pedal, etc., etc., method. Plus, with your method, *you* get to drink all the beer (unless you have a friend there just to keep you company).
David Breneman

Some modern cars have a rubber diaphram that expands as the fluid level goes down due to wear, rather than a vent hole. The thinking behind this is that it would be opened every three thousand miles, at the regular check to equlize pressure and be topped off if necessary.
R. L Carleen

Even the pleated rubber diaphragm within the Master Cylinder only helps to hold back the moisture ingress.

I once read a report where they sealed samples of DOT-3 brake fluid within commercial rubber brake hoses, then tested them for moisture content over the span of several years. The fluid was actually sucking moisture out of the air, THROUGH THE RUBBER HOSES!

Aint't Mother Nature wonderful? - Cheers - - Alec
Alec Darnall


Where in Arkansas are you located? I am in Little Rock. Nice to see other folks from Arkansas on the board.
Richard Smith

This thread was discussed between 03/04/2005 and 07/04/2005

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.