How to diagnose brake failure-om603 - PeachParts Mercedes-Benz Forum

sethro · #1 06-13-2011, 08:46 PM

So,

Wife gets in our 1987 300TD, starts it up (after it sat all night) and proceeds to make an immediate 3 point turn. Vehicle has been running for approx 20 seconds. She applies brakes, nothing there. Wife crashes into steep driveway at approx 5mph. Unbeknownst to her, DESTROYS the radiator. THAT is the reason that I'm buying a new head/gasket.
So, why did the brakes fail? It was only at/immediately after startup, and the brakes have worked flawlessly until the car blew up.

What gives? Vacuum pump?

What say you o collective wisdom?

Thanks in advance,

Seth
Oakland,CA

aaa · #2 06-13-2011, 09:11 PM

If nothing there = pedal sunk to the floor, then it is hydraulic (maste cylinder, a caliper, whatever). Vacuum pump is a hard pedal, I don't think she'd have trouble stopping at 5mph.

compu_85 · #3 06-13-2011, 10:09 PM

If all the vacuum is used up it could be very hard to stop the car. Step 1 is to look at the master cylinder and see if there is still fluid in it.

-J

leathermang · #4 06-13-2011, 10:15 PM

Some cars have an adjustable rod in the master cylinder.. which interfaces with the power booster...
If the vacuum is gone it can be very hard to stop the car at any speed... which I found out almost rolling through a stop sign when my vacuum ruptured... one problem is that in normal seating position you are not able to put max leg force against the pedal ... you have to reposition slightly towards the pedal and pull on the steering wheel.. it can be that hard to push... then if that adjustable rod is not set perfectly... it can be impossible with the vacuum out to stop the car...

sethro · #5 06-13-2011, 11:21 PM

Quote:

Originally Posted by compu_85

If all the vacuum is used up it could be very hard to stop the car. Step 1 is to look at the master cylinder and see if there is still fluid in it.

-J

My bad-brake fluid level is perfect.

leathermang · #6 06-13-2011, 11:31 PM

Tell us again the reason you are buying a new head and gasket .... ?
The radiator messed up the head ? Can't visualize the big picture yet...

Skippy · #7 06-14-2011, 12:20 AM

So to clarify, did the pedal go to the floor, or was it hard?

sethro · #8 06-14-2011, 01:33 AM

Quote:

Originally Posted by leathermang

Tell us again the reason you are buying a new head and gasket .... ?
The radiator messed up the head ? Can't visualize the big picture yet...

#14 head, overheated once due to low coolant.(didnt fill it on an incline) filled up to proper level. Found oil in the coolant (a lot) Car didn't overheat, so continued to drive for a few weeks till we figured out what we were gonna do. Fast forward, wife crashes into steep driveway and loses coolant due to cracked radiator. She gets on freeway, drives the two miles to work. Again,she didn't realize that the coolant was GONE. When she exited freeway, steam is pouring out of hood. Pulls over and shuts off vehicle. I inspect carnage, oil in the coolant reservoir, broken fan blade, cracked&EMPTY radiator. Oil and coolant EVERYWHERE. bad, bad, bad.

So, there ya go.

sethro · #9 06-14-2011, 01:39 AM

Quote:

Originally Posted by Skippy

So to clarify, did the pedal go to the floor, or was it hard?

Just confirmed with wife, pedal went to the floor with NO resistance, POW, then brakes worked fine. She stopped hard five or six times in the next block and the brakes worked great.

Skippy · #10 06-14-2011, 01:49 AM

I see. With no loss of fluid, that would point to the internal seals in the master cylinder being toast. It's a little odd that it worked fine afterward, but I would definitely rebuild or replace it.

leathermang · #11 06-14-2011, 07:44 AM

Quote:

Originally Posted by Skippy

I see. With no loss of fluid, that would point to the internal seals in the master cylinder being toast. It's a little odd that it worked fine afterward, but I would definitely rebuild or replace it.

Indeed a little odd situation......
Seals seldom fix them selves on that kind of part...
I am thinking some intervening variable is at work...and I have no clue as to what it could be ... unless a check valve in the vacuum lines in conjunction with the brake booster did/didnot work temporarily ... Like I mentioned above... if the rod adjustment is short... and the power booster does not work... and someone not familiar with " all the way to the floor " is reporting the incident ... there is room for the alternate theory.... that is not a complaint about your wife's report....as very few people ever experience full to the floor brake pedal....and 90 percent to the floor will feel like 100 percent to the floor to most everyone... and it happens so quickly that few would be concentrating on accurate distance evaluations as compared to the shock and fear of the moment... which is considerable...

compu_85 · #12 06-14-2011, 10:42 AM

Perhaps something with the ABS screwed up?

-J

vstech · #13 06-14-2011, 12:08 PM

ooof.
that much engine carnage, I'd change the entire motor, not just the head...
I think Master cylinder myself. often they fail to respond on first or slow press, but function fine after that.