Master cylinder flakey from fluid temp?
 

Hello All,

1983 300D...

I have a problem with my brake pedal heading right toward the floor after about 30 minutes of stop and go driving. Once that starts, I can "pump it up" to the usual pressure at each stop. The car stops okay even when the pedal goes way down.
It has all the signs of a bad master cylinder but strangely, when you let the car sit for 5 minutes (or overnight), or you drive on the highway for a few minutes without using brakes, it behaves normally again for 30 minutes.

When I first picked it up 5 months ago, it needed both flexible front brake hoses. When I replaced them, I used Valvoline synthetic "DOT 3 and 4" fluid which was the only DOT4 available in the local parts store.

It seems like my temporary failures could be related to the heat of the fluid. I stop a lot, the fluid heats up, pedal to the floor. Let it cool a few minutes, no problem.
My question: Is it possible that I used a brake fluid that doesn't hold up to the MC's needs? Do you think if I took the car to the dealer and had them flush and replace with official MBZ fluid, my problem would disappear?

Thanks,
Garrison

I do not think it is heat related... but it would be good to be sure you are using MB certified brake fluid...
Having them change out the fluid I think would not change your problem..
You might need to be checking out your brake BOOSTER ..... and the vacuum feed supply to it.... intermittent problems are a bummer...

I am thinking you might have air in the system. If what Leathermang suggests in the previous post doesn't cure it, bleed the whole system again. Any time you have a brake pedal that goes to the floor, you have a real problem.

Even cheepie DOT3 fluid would be fine for street driving in one of these cars.

-Jason

And I believe that would qualify...
I am just referring to those that might harm the seals inside the system....
as with using the wrong fluid for the SLS system... ( according to threats by MB mechanics)...

What's the condition of the pads? Worn pads would make the pedal go further down, but not all the way down which could be worn MC or air in the brake lines.

When you replaced the front hoses did you bleed the air out? Make sure the 8mm caliper bleed screws are closed.

Yeah when I replaced the hoses I had it bled at a shop because I didn't trust me and my buddy to do it perfectly. I let it gravity bleed for a bit, did some pedal bleeding, then took it there.
Not sure about air though - I this started happening 3 months after changing those lines, and is intermittent based on time in stop & go traffic. Wouldn't air affect braking all the time? Pads would affect pedal travel all the time. In fact I posted this because I suspect a failed MC would also affect pedal travel all the time.

I can't explain the intermittent nature of this one. I'm going to check for vacuum issues when the weather warms up a little bit more.
Maybe I have sufficient vacuum at highway speeds, but maybe my pump is not putting it out in sufficient quantities when the engine spends more time at idle? (That is, at stop lights...)

Anyone know how much vacuum is required to power the brake booster? I apparently have enough to work the engine shutoff, etc. I do have a leak somewhere though, as I can lock my doors but it leaks out by the time I come back to unlock them.

Ok.. .now we are getting somewhere... you need to trace down and fix that leak..

old fluid could also be the culpret, water absorbed in the system can boil in really hard braking. the shop may have used fluid from an open can or did not completely flush it

As the above post mentions. A contributing factor could be a dragging caliper heating the fluid and any contaminates in the fluid to steam. Then the brakes might act as you describe.

Take a drive and check the temperature of each wheel rim after say ten miles. All wheels should be the same temperature. I would want to flush the existing brake fluid anyways in your case.

You do not have to bleed the brakes to do it. Just have someone reliable to keep adding fluid to the master cylinder as you open each bleeder screw and let the old fluid run out till new fluid is evident.

The individual adding fluid has to be made aware. The back part of the master cylinder must be observed as spillage from the front portion of the master when pretty full travels over into the rear chamber. If he lets the back part of the master cylinder go dry then you would have to bleed.

Your bleeder screws should also be fairly easy to loosen as the car was bled by the garage not that long ago. When finished bleeding any car of mine for the first time I smear the external area of the bleeder screw and slip a tight fitting piece of rubber fuel line over it and fill it with grease.

Badly rusted in place bleeder screws up here in the rust belt are a non issue on any particular car after than. Cost and time to do this is really of no cosequence..Makes a two year flush cycle of the fluid so much easier the next time.

Pedal to the floor!!! Danger Will Robinson... If your pedal goes to the floor, your car is not safe to have on the road.

:o a weak vacuum supply to the brake booster will cause a firm pedal from lack of assist, fluid is un-likely unless there is air trapped in it somewhere. Buy a brake master and get it over with.

And I believe that was mentioned in the first answer to the original post.....
LOL