300E brakes won't give me a break!
 

Hi folks

Hoping someone can save me some time and money here

The brakes in the 300e (1988) were working fine one day, get in the car the next morning and whoosh! Pedal went all the way to the floor! They still 'work' in that they stop the car with the same power - but there is no resistance in the pedal whatsoever. At lights it pushes to the floor easier than the accelerator and the last 1/3rd is loose/you can hear air rushing around somewhere.

Anyway, no leaks or air leaks to be found, fluid full, master cyl replaced about two years ago. Today - it got worse. Stopped at some lights and when I took off, the brakes stayed on! So whatever it is, it's getting worse in the two times I've driven to see what the go is.

Any help is always much appreciated!
Luke

I think it has to be the master cylinder. Nothing else would let the pedal travel so far without a leak. The air sound is probably the brake booster making noise at such excessive pedal travel.

Make sure both the front and rear chambers of the master cylinder are full of brake fluid. I'm sure you realize it's unsafe to drive your car in the current condition.

Both chambers of the master cylinder are full of fluid.

Unsafe is an understatement.... Suicidal is more appropriate I'd say.

As mentioned above, there isn't much in that brake system beyond the mastercylinder to cause the problem you describe. Have you bled the system right through on all wheels?

Is their a chance it is the booster? Or rather a seal somewhere? The fluid system is running fine hence the brakes operate correctly in regards to stopping power - rather the force required to operate them is considerably less.

Had the car towed to my mechanic today and he is under the impression that the master cylinder is fine - and suggested the booster was at fault.

He's a good mechanic but he's been wrong before!

The booster has failed
 

The critical statement is: you can hear air rushing around somewhere.

This would be a broken diaphragm in the brake booster.

.

Any update? Can a booster failure cause the excessive pedal travel like that?

Answer
 

Depending on how it fails; YES.

.

Please share how, because I've never heard this. The brake actuating rod goes straight through the booster, so unless that breaks or bends, I don't understand how.
A broken diaphragm means loss of boost assist, resulting in hard to stop. If somehow he got more air assist than expected due to some other failure, this shouldn't affect pedal travel.
I would certainly bleed first to get a firm pedal at engine off before replacing the booster (unless its failure can be identified).

Not sure exactly the name of the "control valve" used in ATE/Lucas brake booster but briefly the concept is as explained in the "how stuff works" excerpt:

"On cars with a vacuum booster, the brake pedal pushes a rod that passes through the booster into the master cylinder, actuating the master-cylinder piston. The engine creates a partial vacuum inside the vacuum booster on both sides of the diaphragm. When you hit the brake pedal, the rod cracks open a valve, allowing air to enter the booster on one side of the diaphragm while sealing off the vacuum. This increases pressure on that side of the diaphragm so that it helps to push the rod, which in turn pushes the piston in the master cylinder. As the brake pedal is released, the valve seals off the outside air supply while reopening the vacuum valve. This restores vacuum to both sides of the diaphragm, allowing everything to return to its original position."

So, what appears to have happened in your case is the spring that operates the vacuum "control valve" that is located inside the booster chamber has failed which not only reduces the effort required to apply the brakes (probably fully opened valve when you apply the brakes) but also allows the brakes to stay applied because the control valve is not returned to the closed position as the brake pedal is released. Mark

I'm bumping this, as I think I have a similar problem and I'm wondering how this was resolved. Thanks