First let me apologize for the length of this post but I’m hoping it’s going to help out a few people.
I've put together a report and a DIYer pertaining to the morning hard brake pedal issue found in 3 series e9x up to Sept 2007 (See ‘The Trouble’ section below). There's not a whole lot of help out there on the topic and thought I could share some of my experience as I now no longer have the issue. The best part is that the fix is something that can be done without expensive repair work to your vehicle and without disconnecting components that are going to trigger any codes. It just takes a bit of time to physically get to the problem area and a lot of patience. Also, I can’t guarantee if this will work for you. It all depends on how well you follow the procedure and there’s a chance that the problem could stem from something else. A good way to find out if your vehicle is a likely candidate for the fix is to perform ‘The Test’ section below. If you have the time, you may want to go ahead and perform the DIY anyway for the few dollars it’ll cost you.
The Trouble:
Many forum members have experienced a change in the feel of the brake pedal and perhaps some difficulty in starting up their vehicle in the mornings because the pedal wouldn’t extend far enough down to trigger the brake pedal switch. There are complaints that the pedal becomes significantly harder to press down on when the car has been parked for long periods of time. Some of you with the manual trannys that have parked on an incline have not only experienced the hard pedal effect but also the loss of assisted braking capability and found yourselves struggling to stop the car from rolling by employing the hand brake. Drivers of automatics obviously do not rely on braking for the purpose of holding the vehicle in place prior to engine start up but like the manuals, do need the pedal to extend far enough down to start the vehicle. Don’t let anyone tell you that it’s supposed to be that way. It’s not!
The Vacuum and Braking Basics:
With the engine on, air is constantly being drawn out of your car’s brake booster via a vacuum line which runs from the booster housing to the engine. Basically the vacuum creates suction on a diaphragm within the brake booster which lessens the force required on the brake pedal used to operate the master cylinder. When the engine is turned off, a check valve mounted in the vacuum line at the front of the brake booster prevents air from rushing back in allowing the booster to maintain a vacuum and can give the driver up to 4 good normal pumps of the brake pedal either to stop the car in case of engine failure while driving or for those with manual transmissions, to prevent the car from rolling as they prepare to start their vehicle. Any loss of this vacuum makes the brake pedal feel harder than usual to the touch and stopping although possible will take longer to achieve which could result in an accident. As mentioned in the previous paragraph, loss of vacuum can also make starting the engine difficult because more force is required by the driver to push the pedal down far enough to trip the brake pedal switch.
The Culprit:
Below is the related SIB. The trouble is that it doesn’t go deep enough into solving the problem. In fact for me I find it somewhat misleading, making the reader believe that the problem lies within the actual check valve/hose assembly which in my case was not true. In fact the culprit ended up being the little rubber gasket P/N 34336765316 that was supposed to allow the valve/hose assembly to fit snuggly into the orifice in the brake booster housing. I believe slight shrinkage and/or hardening of an undersized gasket combined with vehicle vibrations over time led to it loosening and ultimately a loss of vacuum around the sealing surface; The leak being so slow that it could only be detected by way of the brake pedal of vehicles parked over several hours usually over night. If this was the case, the degree to which shrinkage/hardening occurs could vary with each vehicle. In essence, some leak some don’t. BMW does not directly make mention of this gasket in the SIB. Repair instruction RA 34 33 051 referenced in the SIB calls for a sealing ring to be replaced ‘if necessary’. Hmm! Also, my local dealership couldn't say whether or not the gasket had been resized for these vehicle years.
SI B34 06 07
Brakes November 2007
Technical Service
SUBJECT
Additional Effort Required to Press Brake Pedal Prior to Engine Initial Start
MODEL
E90, E91, E92, E93 (3 Series) produced up to 9/07
SITUATION
A customer may report increased brake pedal effort prior to engine start. This situation can occur if the vehicle sits for extended periods of time. Although the brake pedal can be depressed, the customer may note a change in pedal feel.
CAUSE
The vacuum check valve, fitted at the brake booster, may leak due to component tolerances within the valve assembly.
CORRECTION
Replace the vacuum valve and hose assembly for the brake booster; refer to Repair Instruction RA 34 33 051.
PARTS INFORMATION
Part Number
Description
Quantity
34 33 7 577 336
Vacuum Pipe
1
WARRANTY INFORMATION
Covered under the terms of the BMW New Vehicle Limited Warranty.
Please refer to the latest KSD for all applicable labor operations and allowances.
If the appropriate labor operation is not contained in KSD, then a work time labor operation should be used.
Defect Code
34 33 00 14 00
[ Copyright © 2007 BMW of North America, LLC ]
The Test:
Note: Keep in mind to use a soft touch and a keen ear as you perform this.
1. Remove the black plastic cover from directly over the brake booster unit/brake fluid reservoir area. (Under the hood near the back in line with the steering wheel) The cover as two clips and a rubber tie down.
2. Please Note: Do Not Step On The Brake Pedal At Any Time During The Remainder Of This Test!!! Manual trannys make sure you’re in neutral and apply your hand brake. Start the engine and after about 10 seconds turn it off. This is to ensure that a full vacuum exists within the brake booster.
3. Use a trouble light to locate the check valve on the front of the brake booster. You’ll find it tucked just below the black cylindrical component of the DSC hydraulic unit.
Note: Don’t let the maze of hydraulic lines deter you from doing any of these steps.
4. Use a long flathead screwdriver or reasonable facsimile to reach down between the hydraulic lines and gently press on the rubber gasket located between the check valve and the booster. Push near the gasket’s edge closest to the booster. You should be able to hear a hissing sound air as air enters through the seal. If not, push just a bit harder trying not to damage the gasket or mar the surface booster orifice. I only had to push lightly before I heard air.
5. To confirm your findings, apply moderate pressure to the check valve using the screwdriver tip to see how the gasket responds. Does the gasket appear somewhat loose and can you hear air? If you heard air in parts 4 and/or 5 then go directly to the DIY below.
The DIY:
Most of this procedure is made up of steps just to get access to the rubber gasket. The area around the DSC hydraulic unit/ brake fluid reservoir/ brake booster is congested to say the least and getting a hand down to the lower half of the brake booster where the gasket resides is ‘almost’ impossible without having to completely disconnect hydraulic lines and remove the DSC unit. BMW calls for complete removal of the DSC unit first but we ain’t goin’ there!
You’ll find this DIY somewhat lengthy but I wanted to throw in plenty of detail as pictures are few and far between. Essentially we need to get to the rubber gasket which is located on the brake booster and apply a good size bead of gasket making material around the outer edge. Then we need to get a hand in there to ‘gently’ press the bead to make an air tight seal with the booster front. Too much pressure on the bead will only thin it out. Please, please, if you discover easier ways to do this fix then by all means feel free to share the details.
Also, plan to do this procedure when you can let your vehicle sit afterwards to allow the gasket material to cure. Find out the drying time on the back of the tube of gasket making material.
Let’s get to work!
I’m using steps 1 through 5 part of txusa03’s DIY procedure for changing spark plugs to remove the cowling under the micro filter housing.
http://www.e90post.com/forums/showthread.php?t=174217 (Thank You txusa03) His third picture is a nice shot of the DSC hydraulic unit/reservoir/brake booster compartment.
This will give you full exposure to the brake lines and check valve/hose assembly, etc. He lists tools that will be required. You obviously wont be needing all of them. In addition you will need:
- One tube of gasket maker (I used flexible non-hardening Permatex Black … not sure what it’ll do to the paint on the booster though) You may know of a better material for this purpose.
- Long shank flathead screw driver
- 10mm socket with extension bar
- A heavy gage wire coat hanger
- Trouble light
- Ideally a small hand with a long arm
Step 1. Remove the micro filter. Use the 5/16 hex socket and remove 6 screws and just lift the micro filter housing out.
Step 2. Remove both covers on each side of the micro filter. Each one of these cover has 2 clips and a rubber tie down (red circle), locate and pop both clips loose, then pull the cover up and out of the rubber tie down. With these covers removed, you should see another hex screw (yellow circle) that hold the cowling in place. Remove these last two hex screws as well (one on each side).
Step 3. To remove the cowling you need to pull out the rectangular harness that holds the wiring.
Step 4. Behind the rectangular harness you will see additional wire tubing held on the underside of the cowling. Unclip those 3 clips and pull the wire tubing down
Step 5. Before you remove the cowling, unhook any other wiring that is still attached to the top side of the cowling. Now pull the cowling straight out toward the front of the car and tilt it up a bit as you pull (it helps to wiggle and work each corner loose as you go along).
Step 6. Removing the black plastic bulkhead (located between the engine compartment and the brake booster compartment). Disconnect the brake booster check valve/hose assembly at this location. Pull the end of hose through the bulkhead. Next, carefully remove the bulkhead by pulling on the four tabs that hold it in place, swing it up and separate it from the brake line by pulling away.
NOTE: Do not disconnect any wiring to the DSC hydraulic unit when performing the next steps or unless you want to trigger a code. Also, do not disconnect any of the hydraulic lines from the DSC unit.
Step 7. Unbolting and raising the DSC hydraulic unit. This allows you to gain access to the rubber gasket with your fingers and also allows more room to remove the check valve/hose assembly from the booster. Use the 10mm socket and extension bar to remove the three DSC hydraulic unit hold down nuts. Be carefully getting them out. A magnetized socket would be best. Find an old metal coat hanger to have at the ready. Roughly configure the hanger to suspend the raised DSC unit from the underside of the hood. There’s a small hole in just the right place in the hood. The brake lines are flexible enough to allow the DSC unit to be lifted up and over the hold down bracket bolting. Some of the lines have flex hoses connected which make lifting easier. Please use caution and pull the DSC unit up firmly being careful not to kink any of the attached brake lines. Use common sense. The unit can be pulled a little towards the side of the car as well to help clear a way. Adjust the length of the coat hanger to suspend the DSC unit. Keep it tight in place.
Step 8. Removing the brake booster check valve/hose assembly. Slip the flat head of the long screw driver between the check valve and brake booster and with a twisting motion, gently pry the valve out of the booster. The rubber gasket ‘should’ remain attached to the booster. Warning: Do not remove the gasket. It’s a bitch to put back… I know. Pull the check valve/hose assembly completely out of the compartment and put aside.
Step 9. Applying gasket material to the rubber gasket. Open the tube of gasket maker and attach the long nozzle applicator that should come with it. Apply a bead of gasket material liberally around the circumference of the gasket trying to stay in as close to the booster as possible. Avoid getting any gasket goop on the gasket face. Use the long screw driver to rotate the rubber gasket as you apply the material.
Step 10. Making an air tight seal between the rubber gasket and the brake booster.
Note: this step must be done without rotating the rubber gasket otherwise the gasket bead will not seal properly. By suspending the DSC unit I’ve now created a space just wide enough to slide my right hand in, palm side against the booster. My hand is jammed against the DSC bolting bracket on the wheel well but I can feel the entire rubber gasket with my index and middle fingers. The back of the right hand should almost be touching the back of the DSC unit. Now lightly run a finger around the gasket material to form a nice thick air tight bead while pressing it lightly against the booster and rubber gasket. There is no need to press hard. Just make good contact with the booster.
Step 11. Making an air tight seal between the check valve/hose assembly and the rubber gasket. This is kind of a good measure thing that I decided to do while I had the hose assembly out. Apply a small bead of gasket material to the check valve seat that fits against the rubber gasket. Note: Do not apply gasket material to the barbs on the valve.
Fish the check valve/hose assembly back into position and use the screw driver to push it home.
DO NOT TURN THE ENGINE ON UNTIL THE GASKET MATERIAL IS COMPLETELY DRY. (8 HOURS)
Well that’s it! Time to put everything back together. I think the DSC module hold down nuts require 8 Nm of torque. (6 ft-lbs)
The End:
Good luck everybody. Just think! No more hard brake pedal. Woohoo!