Rebuilt brake system. Result: Too much pedal travel with engine running

[beachbear]

A month ago, I replaced the master brake cylinder. This week I replaced the rubber brake lines; rebuilt 3 of the wheel cylinders, and replaced the left rear cylinder. All pads were replaced. I primed the master cylinder. I bled the brake lines. There are no leaks. With the engine off, I have a firm brake pedal which stops where I expect it. With the engine running, the pedal can be depressed, almost, to the floor. Can anybody suggest a reason?

[Tyler S]

You still have air in the system. If you let the master cylinder run dry when replacing your lines, you will need to bench blead it. There is still air trapped at the end of the piston and the brake rod from the pedal wont travel far enough to expel it while bleeding the system. You can try bleeding the system with the car running or use a vacuum bleeder to rid the air. Start at the right rear wheel first and work your way to the closest wheel (left front). Make sure your helper is depressing the pedal all the way to the floor during the procedure. DO NOT pump the pedal between bleed cycles. This pulls the air back up the system.
If all fails you will need to remove the master and bench bleed it.

[ja17]

Make sure that you did not reverse the front calipers. If so the bleeder is on the bottom instead of the top. In this case you cannot bleed the air from the system. The top stage of the pedal is for the rear brakes. With the engine off, try pumping up the brakes, hold pressure, then have someone loosen the fitting at the master cylinder which goes to the rear brakes. When the pedal goes down tighten the fitting. This will bleed the master cylinder of air. Repeat as needed to expel all air.  Be careful to catch any brake fluid since it can damage paint.

[beachbear]

Thank you, very much for your responses. I removed the master cylinder, primed it (boy did it have air), and voila! Also, I had noted the caliper's orientation prior to removal. It didn't even occur to me that they could be reversed. I could only shake my head as I reviewed my post and noted that I constantly called the wheel calipers "cylinders"

Another issue related to this job: I almost didn't get the right rear caliper off for service. It seems the last garage to service it applied way too much torque to the caliper bolts. The part identified in the illustration as 116: Plate, brake, with nuts was broken at the plate. I managed to find some steel and fashion a repair, but I can't find this as a part for sale anywhere. They also left off the caliper locking plate from both rear calipers.

Can any one suggest a best place to start looking for a replacement plate with nuts?

Edit: found them at kkmfg.com!

[Tyler S]

Tip for anyone that will be doing brake hydraulic work. Push the brake pedal down and lock it into place with a piece of wood or something. This closes the ports to the reservoir and keeps the master cylinder from running dry while doing hoses, calipers etc.. Kind of like putting your finger on the top of a straw. The fluid stays put.
Tyler

[ja17]

Thanks Tyler! Good tip! You just taught an old dog a new trick!

[Mike K]

I went through a similar exercise last year. I redid the whole braking system, new brake lines, stainless steel hoses, 4 new calipers & new master cylinder.

I bench bled the master cylinder prior to installation, it made a huge difference.

For anyone undertaking this job, bench bleeding is a must, here's a link:
http://www.sl113.org/forums/index.php?topic=22964.msg164235#msg164235

Best,

Mike

[Benz Dr.]

There's an eccentric bolt at the push pin just behind your brake booster that can be adjusted. I normally set it so that the notch in the bolt head points towards the firewall. I checked a number of cars at PUB last year and only found one that was set up properly. The notch indicates the highest point of the eccentric and if set up as indicated will give you the least amount of free pedal travel. I have seen where the brakes wouldn't release after adjustment so I had to stop and move the eccentric slightly and all was well after. You will be very surprised how much better your bakes will feel once you have this set up correctly.


 Speaking of brakes:
I could have used them last night before I ended up in the bottom of a ditch, upside down, hard against a culvert. Banged up a bit but was not hurt. ML 430 totaled.

  It's why I drive a Benz. I wouldn't have survived in anything else. I can tell you that travelling at 55 MPH down the bottom of a ditch heading for either a telephone pole or a steel culvert is a very enlightening adventure!  Seeing no safe escape from the bottom of the ditch, and heading straight into a very hard impact, I cranked the steering wheel hard left so the passenger side would take the brunt and that made me flip upside down.
Someone managed to open my door and cut my seat belt to free me as I was still belted in while hanging upside down.

No charges laid. I'm going to part the ML out.     

[Tyler S]

Wow! Glad your ok. Remember its just a car and it probably save your life.

[66andBlue]

Glad to know that you are sore but well Dan; looks like you have another birthday date now.

There's an eccentric bolt at the push pin just behind your brake booster that can be adjusted. I normally set it so that the notch in the bolt head points towards the firewall. I checked a number of cars at PUB last year and only found one that was set up properly. The notch indicates the highest point of the eccentric and if set up as indicated will give you the least amount of free pedal travel. I have seen where the brakes wouldn't release after adjustment so I had to stop and move the eccentric slightly and all was well after. You will be very surprised how much better your bakes will feel once you have this set up correctly.

By coincidence I had to remove the booster yesterday because of a slight leak and took a photo of the bolt to remind me how to install it again.  Perhaps the photo will help others.

[Benz Dr.]

Thanks, I'll be OK. Feeling very sore right now but I expected that.

[Mike K]

Glad you've survived to tell the tale! Wish you a speedy recovery.

Best,
Mike

[ja17]

Glad  your OK Dan, and I hope your shinny-side is always up in the future. Welcome to the "dirty side up club".  I am a four time member (ex-rally car driver).

[rgafitanu@gmail.com]

Beachbear,
About the locking plate 117. A year ago when I serviced the brakes my plate was all mangled so I left it out. I figured that it's another MB overdesign, modern brake caliper bolts do not have even split washers for locking. After a couple of months when I was checking something else I found both bolts loose. So the locking plate is definitely needed. The same thing happened with the drive shaft to differential bolts. Apparently there is serious vibration. Modern bolts have fine pitch which makes them better for locking. You can find the locking plates very cheap at the MB dealer or www.getmercedesparts.com (dealer in Atlanta with internet sales).
Radu

[Benz Dr.]

I torque the caliper mount screws to 80-85 ft -lbs. which is the same amount wheel screws are tightened. Even without the lock plates they should never come loose. Given that most lock plates are used up and broken off when checked, I'm thinking it's better with them but most cars won't have complete serviceable locking plates anyway.

[jukhaa]

Dear Dr. Benz,

I have a new brake booster and new brake hoses (in the front), but the brakes get blocked after a half an hour drive. Can the eccentric bolt position be the help for this. for now it has been in the position which you suggested as optimum, pointing toward the firewll.

I am getting still a new master cylinder soon and will change that ,too.

regards jukhaa

[beachbear]

Oh My Gosh! I was looking around this forum today before I post a new question and thought to check on replies to this one! As my photo shows, the screw was 180 degrees out. It did make all the difference. Thank you!

[Benz Dr.]

Dear Dr. Benz,

I have a new brake booster and new brake hoses (in the front), but the brakes get blocked after a half an hour drive. Can the eccentric bolt position be the help for this. for now it has been in the position which you suggested as optimum, pointing toward the firewll.

I am getting still a new master cylinder soon and will change that ,too.

regards jukhaa

This will happen after you apply the brakes several times if there isn't enough free play at the push pin. Once your brakes start to drag, heat build up will make your fluid expand and then they will really lock up. Stop your car ( that won't be hard to do ) and turn your eccentric forward about a 1/4 turn. This should make your brakes release and you should be able to move your car easily just by pushing on it. If it still won't move, try a bit more adjustment until your car rolls freely.

I'm not sure why but some cars require more push pin free play than others.

[gimp]

Dan
Sorry to hear about your accident.
Glad you are alright.
Frank