Brake question (non MB). - PeachParts Mercedes-Benz Forum

Benz300 · #1 12-31-2005, 07:05 PM

I've got a strange problem with a 95 Mitsubishi Montero truck.
The brakes worked fine till one day when the brake pedal decided to become really spongy and also engages very low (close to the firewall).
After checking all the calipers and pads (they're new), bleeding the system, I decided it was a bad master cylinder.
Changed the master cylinder. no effect. Could've been a bad new M/C so I chaned it yet again. no change.
With engine off, the pedal is solid / hard and doesnt drop at all even after constant pressure. Hence I'm assuming there's no air in the system and M/C is good.
As soon as the engine is turned on, the pedal drops almost 5 or so inches. With the engine ON and car running I can get a high pedal if the pedal is pumped. Given I've bled the system thoroughly atleast 8 times by now, bench bled master cylinder as well, could someone kindly tell me what else could be causing this issue ? It does have ABS but the ABS isnt working due to bad ABS sensors. The brakes worked fine for a year even after ABS wasn't working so I'm assuming ABS is not related to this issue... any suggestions would be appreciated.

Moneypit SEL · #2 01-01-2006, 02:02 AM

I'm not familiar with the abs on this truck, and I damn sure ain't looking it up tonight. However, one thing that can cause a problem similar to yours is loose wheel bearings. Try isolating the problem by (carefully) pinching off the flexible brake lines at each wheel. Once isolated to one corner, it should be easier to find the problem.

Benz300 · #3 01-06-2006, 09:55 AM

bump.
any info will be greatly appreciated

TX76513 · #4 01-06-2006, 10:39 AM

Quote:

Originally Posted by Benz300

bump.
any info will be greatly appreciated

Does that vehicle have a check valve on the booster? It may have stuck while sitting -

Benz300 · #5 01-06-2006, 10:43 AM

wouldn't a stuck check valve give a very hard pedal ?
or are we contemplating the fact that if the check valve is open, then it would leak the air out of the booster giving a very low pedal in return ?
I did not notice a check valve at the point where the vacuum pipe connects to the booster. do some boosters have internal check valves ?
pumping the pedal gives a high/hard pedal. could that be a symptom of faulty check valve or booster ?

Benz300 · #6 01-06-2006, 10:47 AM

have tried three different brand new master cylinders so far without any change. so can't be the master cylinder.
bled the brakes with and without the engine running as well without any change in the pedal moving too far down.

TX76513 · #7 01-06-2006, 10:47 AM

Thinking open and the action that happens when you start the motor. I tend to always look at the smaller parts first and rule them out - you conditions sounds like the booster, but there are other things possible.

Internal check valve - I don't know the answer to that

Here is a picture of a Volvo Check valve

Nate · #8 01-06-2006, 10:50 AM

on the 82 f150 We got this same problem after a new MC... This was after replacing seised calipers, rusted hard lines, squishy ruberlines and bad wheel cylinders... (put away wet/muddy many a time)
After giving up, we took one of the wifes vehicals to the cabin for the weekend. When we came back, the frount brakes were better and we had good pedal feel...
We just couldnt get the backs to bleed, which ended up being a proportioning valve (ugg)

After all the new parts, we had a stubborn air pocket somewhere, and letting it sit helped...

If you need this everyday, I'd call a few shops and see if they have a powerbleeder, and what they charge to use it... If not, let it sit for a day or so, and try again.

Or you could have a bad booster, but I really dont know

~Nate

TX76513 · #9 01-06-2006, 11:14 AM

Quote:

Originally Posted by Benz300

have tried three different brand new master cylinders so far without any change. so can't be the master cylinder.
bled the brakes with and without the engine running as well without any change in the pedal moving too far down.

Have to ask this - did you bench bleed the new MC?

Benz300 · #10 01-06-2006, 11:16 AM

yes, the m/c was bench bled.

Benz300 · #11 01-06-2006, 12:05 PM

been driving it continously for a month after changing the master cylinders.