Need help diagnosing brake problem
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From: Long Island, New York
1994 MB E320, 1988 Porsche 951
Need help diagnosing brake problem
Hi guys, I need some input on how to diagnose a brake problem on a '94 E320.
When park overnight, and I start move the car, the brake pedal drops partway to the floorboard, one pump, and the pedal is fine. On long highway drives, stepping on the brake for a toll booth or exit ramp, the pedal drops partway to the floorboard, one pump and the pedal is fine. Been a reoccurring problems for about six months, I now pump the brake before I stop or slow down. I bled the brake cylinders, there was a little bit of air in them. Brakes fine for a few weeks, then pedal drops again. I then flushed out the brake lines, replaced with Valvoline DOT 4 synthetic, replaced the brakes pads, brakes fine for a few weeks, the same problem again. With engine off, pumping up the pedal and holding, there is no drop in pedal. With enigne on, pumping up the pedal and holding, there's also no drop. There's no loss of brake fluid in the system. Is air being sucked into the lines from somewhere?
When park overnight, and I start move the car, the brake pedal drops partway to the floorboard, one pump, and the pedal is fine. On long highway drives, stepping on the brake for a toll booth or exit ramp, the pedal drops partway to the floorboard, one pump and the pedal is fine. Been a reoccurring problems for about six months, I now pump the brake before I stop or slow down. I bled the brake cylinders, there was a little bit of air in them. Brakes fine for a few weeks, then pedal drops again. I then flushed out the brake lines, replaced with Valvoline DOT 4 synthetic, replaced the brakes pads, brakes fine for a few weeks, the same problem again. With engine off, pumping up the pedal and holding, there is no drop in pedal. With enigne on, pumping up the pedal and holding, there's also no drop. There's no loss of brake fluid in the system. Is air being sucked into the lines from somewhere?
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Joined: Dec 2006
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From: Bellevue, NE
1995 E320 SE, 162,000 Miles (Sold) 2007 CLK350 Cabriolet 2014 ML550
You need a new master cylinder, or to rebuild the old one. The fluid is leaking by the seals in the master cylinder and going back to the reservoir, rather than pressurizing the slave cylinders at the brake calipers. It's not air or an exterior fluid leak. Here's a link to buy a new one.
http://mercedes.thebenzbin.com/?year...Field22222.y=8
http://mercedes.thebenzbin.com/?year...Field22222.y=8
Last edited by shdoug; Apr 3, 2007 at 08:34 PM.
Thread Starter
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From: Long Island, New York
1994 MB E320, 1988 Porsche 951
Brake pressure holds when I'm applying heavy pressure on pedal, wouldn't pdeal drop if the master cyl is leaking? Is it a big job to r/r a master cyl on the E320? Thanks for comments, this forum the greatest!
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From: Northern Cali
W124
Yes it is kinda involved when doing a master cylinder replacement simply because of its location in the engine bay. If it isn't your master cylinder then I'd begin to worry because it could be some ABS components and those are a biatch to diagnose and are expensive.
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From: Bellevue, NE
1995 E320 SE, 162,000 Miles (Sold) 2007 CLK350 Cabriolet 2014 ML550
Sorry, I didn't read carefully enough. It's probably not the master cylinder. Something is causing one or more of your caliper pistons to slide back or be pushed back in to the slave cylinder partially when your foot is not on the brake and lose contact with the brake pads. The first pump of the pedal pushes the piston back out to where it touches the pads. The second pump applies pressure and squeezes the caliper. This would explain why it holds after the first pump and when the car is not moving.
I've had this happen on many cars, but have never found the cause. I would think maybe a warped disc or loose caliper mounting could cause this, but I'm not sure. Also make sure the reservoir is not in a vacuum causing the pistons to be sucked back in. I don't think this is likely, but I'm grasping at straws and it it a completely air-tight system.
I've had this happen on many cars, but have never found the cause. I would think maybe a warped disc or loose caliper mounting could cause this, but I'm not sure. Also make sure the reservoir is not in a vacuum causing the pistons to be sucked back in. I don't think this is likely, but I'm grasping at straws and it it a completely air-tight system.
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From: Long Island, New York
1994 MB E320, 1988 Porsche 951
Since the brake pedal holds when pressure is applied to it, it might not be a leaky master cyl. I lose all pedal pressure when parked overnight till I step on the pedals once to build pressure; is it possible the brake booster line/hose is leaking vacuum when parked for extended period? I also lose partial pedal pressure while driving on a highway, would that eliminate the leaky brake booster line/hose idea since highway engine speed vacuum would compensate for leaking booster hose?? How would I check if the booster hose is leaking vacuum? The brake booster itself seems to be fine, pedal pressure to stop car feels normal (once the pedal pressure is restored). All inputs are deeply appreciated. This thing is driving me nuts. I hate to restort to the r/r everything till the problem is solved technique (too expensive).
