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+--- Thread: Brake pedal gets soft the longer I drive. (/showthread.php?tid=3130)
Brake pedal gets soft the longer I drive. - fehron - 11-15-2011
The brake pedal gets soft the longer I drive. The brakes work great(very firm) when we first leave the house, but then after four or five miles or so the pedal gets increasingly softer. This is an 82 240d that's been sitting for a couple of years. Besides bleeding the brakes, what else would be the first suspicions?
Also the engine doesn't shut off with the key(sometimes, you can rev the engine and shut the key off), I understand this is a vaccum issue, but I wouldn't expect this combination of problems to be connected, could they be related. I guess I just want to have any parts needed when I get a chance to go back and work on it.
RE: Brake pedal gets soft the longer I drive. - sassparilla_kid - 11-15-2011
The brakes on these cars are also vacuum assisted, so a vacuum leak could potentially cause problems
RE: Brake pedal gets soft the longer I drive. - fehron - 11-15-2011
But, would it cause a soft pedal that continues to get softer the longer it drives?
RE: Brake pedal gets soft the longer I drive. - willbhere4u - 11-15-2011
Yea with out vacuum they are hard! after it gets full vacuum it it gets softer
RE: Brake pedal gets soft the longer I drive. - aaa - 11-15-2011
Soft pedal would usually be hydraulic issues. Start with replacing the master cylinder.
RE: Brake pedal gets soft the longer I drive. - sassparilla_kid - 11-16-2011
Could water in the brake fluid cause them to get softer? After a little while of braking wouldn't it get hot enough to vaporize and cause bubbles in the lines? Just a thought. I know the brakes on my friend's diesel rabbit went out on us while we were driving to the hardware store one day, but that just happened because one of the lines had become loose and it was losing pressure through it
RE: Brake pedal gets soft the longer I drive. - willbhere4u - 11-16-2011
Is it a soft pedal? or just softer than having no power breaks?
IE soft/spongy goes to the floor?
or is easy to use like power breaks are working correctly ?
RE: Brake pedal gets soft the longer I drive. - fehron - 11-17-2011
I'm actually loosing the brakes. I wasn't going fast, but after 3 or 4 pumps, I hit the parking brake and continued to pump. never really accomplishing much with the pumping.
I'm gonna go back down on thanksgiving weekend and start with a good bleed to purge all the lines and go from there. I wouldn't be surprised if autozone and oreillys don't stock a master cylinder for this car, so if necessary I may pull the master cylinder off my dads wrecked 300. It worked fine when it tangled with a white tail last year.
RE: Brake pedal gets soft the longer I drive. - willbhere4u - 11-18-2011
there is an O ring between the master and the booster that needs to be there to seal master to the booster or you will have a huge vacuum leak
RE: Brake pedal gets soft the longer I drive. - fehron - 11-18-2011
But, would a loss of vacuum cause a soft pedal? I would expect if it were a bad booster or loss of vacuum, the pedal would get harder to push.
I do have a vacuum problem that hasn't been diagnosed yet as far as it being the pump or a leaky connection in one of the vacuum hoses.
Thanks for all the input, I can't wait to get this thing straightened out and on the road.
RE: Brake pedal gets soft the longer I drive. - fehron - 12-29-2011
Well, I guess I have an update.?.? I believe the brake booster is bad. I attached a vacuum tester to the brake booster and it doesn't even think about maintaining a vacuum. I've driven it with the brake booster detached from the vacuum system and it feels exactly the same. I will replace the booster and report back. The vacuum pump is functioning properly according to the gauge on my vacuum tester.
As for it not shutting off with the key, that's also directly related to vacuum leaks in addition to the brake booster. Somewhere in the mess of vacuum lines that run into the firewall, there is a major leak. By bypassing straight to the brown line that goes to the ignition switch it shuts off every time without hesitation. I understand that those other lines are for the power locks. And the cruise control? Regardless, I have a Benz that starts and shuts off when commanded.
RE: Brake pedal gets soft the longer I drive. - Hercules - 12-29-2011
If no brake fluid leaks,replace master cyl.(bleeding by). Remember brake fluid reservoir has multiple compartments,All must be filled. Brake booster if defective will cause high hard pedal. Good luck.