brake pedal went to the floor while braking!!?? - Page 2 - PeachParts Mercedes-Benz Forum

kerry · #16 06-03-2009, 07:33 PM

Can you explain how a frozen caliper would cause the pedal to go to the floor? I don't see the connection. Even if both rear calipers failed by leaking, you still should have front calipers and a hard pedal.

MTUpower · #17 06-03-2009, 09:36 PM

I had the same thing and it had nothing to do with the MC. It was a sticky caliper. The caliper sticks and heats the rotor- which heats the caliper, which heats the brake fluid. The fluid boils. Gas is compressible, liquid is not. Step on the pedal and you are compressing gas, not fluid (liquid). Hence you step all the way to the floor and get little to no stopping power. Later on the fluid cools and voila! your brakes appear fine- until you drive enough to heat the brake fluid to a boil and then no brakes. When this happens stop if you can and exit the car and feel the front rims- one ought to be to damn hot to touch. That's the caliper to replace. Or change to w126 genI/genII setup and never have this issue again. One thought about this happening with my family in the car was all it took to convince me to install vented rotors which cool a million times better than the solid rotors of the w123.

kerry · #18 06-03-2009, 09:56 PM

Ah, I get it. Since the fluid becomes a gas, does it expand enough to boil out of the reservoir?

MTUpower · #19 06-03-2009, 10:35 PM

Quote:

Originally Posted by kerry

Ah, I get it. Since the fluid becomes a gas, does it expand enough to boil out of the reservoir?

I believe it boils in the line or in the caliper. Drive the car when cold for a bit and before you lose your brakes stop, get out and put your hand on the rims so you can see if they are the same temp. If one is really hot then you have identified your problem. All it takes is one spot to boil and then the fluid/pedal fails.

mach0415 · #20 07-08-2010, 12:36 PM

I just had this happen to me, but upon cold soak, the problem does not rear its ugly head. After braking a few times, pedal starts to become spongy, go to the floor and pump up if needed. No leaks and M/C is full. I am thinking 2 possibilities. 1) Most commonly going to be a M/C with an internal seal leak. 2) Water in fluid, causing gassing upon hot brake fluid. What do you all think? This fluid is dirty, but not the worst I've seen. However, it could have moisture accumulation.

barry123400 · #21 07-08-2010, 01:02 PM

Feel the rims. If not abnormally hot after a mile or so drive with no application of the brakes change the brake fluid. Problem still present change the master cylinder. Usually a bad master cylinder is consistantly bad once defective. I guess there can be exceptions.

Actually brakes are nothing to fool around with. Positive identification of the problem is always needed. Too much is at stake otherwise.

I once lost my brakes temporarily on a long downhill stretch from a very high bridge. Got enough by pumping to get stopped eventually. Just before I was going to drag the body along the side concrete wall to kill off energy.

This was way back before the dual circuit master was put into use by manufactures. I have always found it only a minor improvement when one side dropped out. Better than nothing but not much.

mach0415 · #22 07-19-2010, 03:23 PM

I suspect there was some water from condensation in my brakes. I flushed the system last week and I have not had any concerns after. Hot brakes will raise the temp of the fluid quickly. If there is water in it, it will gas and compress until cool again. Brake fluid can take on a significant amount of water through condensation over time. Since DOT 3 and 4 is hydroscopic, water molecules are encapsulated by the polyglycol molecules. This is one other reason to do regular maintenance on brake fluid. FWIW