[Submariner409]

Never heard something like this. First the reservoir of the master zylinder is not airtight, because temperature, wear of the brake pads which forces the pistons in the calliper to move out, which leads into more needed brake fluid in the system and therefor less fluid in the master zylinder, leads into change of fluid quantity in the master zylinder. If the master zylinder would be airtight the system would not work by design. To much fluid would simply come out at expansion. That could not cause this problem. Lower grade brake fluid in the system? How likely is that? More likely is, that the fluid is old and full of water...

The brake master cylinder cap IS airtight. If you look closely you'll see the cap gasket is actually a rubber bellows that flexes in and out with fluid use. The metal cap itself has a tiny breather hole to allow air to the outer side of the bellows. I'll bet some members have removed the cap, seen the bellows flexed downward and wondered why ........... it's designed that way. You simply check/top off fluid level and push the bellows back into place.

Why the sealed bellows? Because brake fluid is hygroscopic - it LOVES water and will absorb it from the humidity in the atmosphere. Brake fluid should - should - be exchanged once every 5 or 6 years.

automobile brake master cylinder cap bellows images - Google Search

 

[Nonickatall]

The brake master cylinder cap IS airtight. If you look closely you'll see the cap gasket is actually a rubber bellows that flexes in and out with fluid use. The metal cap itself has a tiny breather hole to allow air to the outer side of the bellows. I'll bet some members have removed the cap, seen the bellows flexed downward and wondered why ........... it's designed that way. You simply check/top off fluid level and push the bellows back into place.

Why the sealed bellows? Because brake fluid is hygroscopic - it LOVES water and will absorb it from the humidity in the atmosphere. Brake fluid should - should - be exchanged once every 5 or 6 years.

automobile brake master cylinder cap bellows images - Google Search

I disagree, if I may.

The bellows are made for the braking process. When you brake, the master cylinder forces fluid into the system and the pistons out. The bellows compensate for the falling liquid level. Because it has to be quick. The bellow also prevents too much air from penetrating/pumping into the system and thus too much humidity from entering.

The hole, on the other hand, is there because air muss go into the system in the long term. Because the brake pads wear out, become thinner, the pistons have to come out further and this leads to a sinking fluid level, which is much higher than when braking once. This must be compensated for, otherwise a vacuum would be created in the system, which would counteract the brakes. This air goes trouh the rubber seal.

And water always gets into the system, which is exactly why you don't use hydraulic oil but brake fluid, which has to absorb this water in a targeted manner to prevent vapor bubbles from forming, which would cause the brakes to fail. All German vehicle manufacturers therefor prescribe a change interval of 2 years.

In my humble view, you don't have to change the fluid every two years, it also depends on how much the car is driven and where. It's definitely different in sunny California, than in Canada.

You can see it in the liquid color. Fresh brake fluid is clear as water, but as it ages and absorbs water it turns yellow and larter brown. It should never turn dark brown, like motor oil, as the water will also cause excessive corrosion in the braking system.

 

[Submariner409]

Wrong. In most of our ABS-enabled vehicles the entire system (and cap bellows) is sealed. The two chambers in the master cylinder are already full of fluid - when the pedal is depressed the captive fluid is forced to pressurize the entire system, the reservoir is closed off, and - since the fluid is incompressible reservoir volume does not fall - the reservoir acts as a - - reservoir - - to replenish any fluid lost by weeps and leaks as the system is used/ages. The only time fluid is added to the master cylinder piston bores is if there is room for gravity replenishment after the pedal returns to its rest position and to compensate for pad wear. Depressing the brake pedal for normal to hard braking moves less than 1 tsp of brake fluid - those caliper pistons move only a few thousandths of an inch - and the master cylinder volumes are mated with the number of pistons in the calipers total.

brake master cylinder cutaway diagram for fluid replenishment - Google Search

brake master cylinder bellows seal function - Google Search

 

[Nonickatall]

Wrong. In most of our ABS-enabled vehicles the entire system (and cap bellows) is sealed. The two chambers in the master cylinder are already full of fluid - when the pedal is depressed the captive fluid is forced to pressurize the entire system, the reservoir is closed off, and - since the fluid is incompressible reservoir volume does not fall - the reservoir acts as a - - reservoir - - to replenish any fluid lost by weeps and leaks as the system is used/ages. The only time fluid is added to the master cylinder piston bores is if there is room for gravity replenishment after the pedal returns to its rest position and to compensate for pad wear. Depressing the brake pedal for normal to hard braking moves less than 1 tsp of brake fluid - those caliper pistons move only a few thousandths of an inch - and the master cylinder volumes are mated with the number of pistons in the calipers total.

brake master cylinder cutaway diagram for fluid replenishment - Google Search

brake master cylinder bellows seal function - Google Search

Ok, if so answer me a few simple questions:

The brake pads move back and forth a maximum of 1mm in each piston when braking, so not much fluid has to be moved in the system, which is what the bellows in the cover is responsible for and we agree. But if the brake pads wear out, it can easily add up to a 1 cm stroke, wich mean on each piston. Let's assume the reservoir was dimensioned so large that it still has liquid when the pads are completely worn down, so that the braking system does not fail but is then almost empty, there would be a vacuum if the system were sealed. And from where should the pressure equalization come from in this case?

That is the function of the resoirvoir, not leakage, because if you have a leakage you have a problem.

And if the system were sealed, why should the brake fluid be hygroscopic?
And what's the point of replacing them at all?

You can also easily check whether your system is sealed. Open a bleed screw on one of the brake calipers. You will notice that brake fluid is leaking and if you wait your reservoir will be empty.

I have my own garage in Germany, I repair brake systems and regularly change the brake fluid. I know what I'm talking about.

I hope you don't get me wrong. I mean that not unfriendly. Just a technical dispute.

Many greetings from Germany

 

[Nonickatall]

Today I was on my car and took the chance to take a look on the cap. If you push the bellow a little you see a cut in the middle. That is where the Air comes in.
So as i said. No sealed system...

I hope you dont get me wrong. Best regards...

 

[Usamestizo]

where does the outside air come in from. I dont recall seeing a hole in the cap. I could be wrong

 

[Nonickatall]

where does the outside air come in from. I dont recall seeing a hole in the cap. I could be wrong

Between the cap and the rubber, than through the cut in the rubber....
Some caps on other cars have as well tiny holes.

 

[Nonickatall]

Now i wanted to know and disassembled the cap.

You see a cutting in the middle to the side. There the Air comes above the bellow and through the cut in the reservoir.

On my first pic you see that cut on the right side.

 

[[email protected]]

I have the same problem with my truck. Had to replace booster because of a power steering leak. Once I replaced the booster the brakes started locking up. I replaced the booster 3 times. I replaced both front calipers twice and it still locks up. I didn't replace the rear calipers yet. Master cylinder was also replaced. Anyone have any ideas?

Did you ever get yours fixed ????