Brake System Help

[Fallinggator]

My brake pedal is travelling way too far at the moment, and it's probably my fault...although it wasn't working before either. It started with my brake pedal being really spongy, so I decided I'd bleed my brakes. In the process I stripped out 1 bleeder valve, and had to buy a rebuilt caliper for the left rear. The other three came out eventually, and I replaced the bleeder valves to be safe. When I was examining the calipers I discovered that it was difficult to push the caliper pistons in even with a wrench, so I took them off the car to rebuild. I'm out of work at the moment, so I didn't have much money to work with. I decided to try to reuse the piston seals...probably not a great idea. I was really careful taking them off, but guessing this just wasn't going to work no matter how careful I was. On the plus side I cleaned up the pistons which weren't too bad and the piston housings, and now the pistons can move with just a bit of hand pressure. I put everything back on the car, and bled the brakes with a mityvac vacuum tool. They seemed well bled, and I took the car for a test drive. Unfortunately the pedal was going WAY too far down. It was activating the brakes, but way too much travel. I was pretty thoroughly beat after doing all this, so I just went to bed. This morning I checked the fluid level, and it was significantly below where it had been. It was right on the Max Fill Line, and now it's down probably 1/2 an inch. Am I correctly assuming that my problem is the seals are leaking, or could it also be a bad master cylinder? The first time I tried to bleed the brakes I used the partner/pedal method, and I'm worried that my partner may have pressed the pedal too far during this damaging the MC seals. Is there a way to determine this before I order the new seals for my calipers? Could there be anything else likely to cause the problem like the ABS? I've got a US 1995 mercedes E320 Wagon W124.092 M104.992.

[Ferdman]

It sounds like there is air trapped in the system. Check that the rear chamber of the master cylinder is full of brake fluid, then bleed all the brake calipers again. It's best to use a pressure bleeder. The sequence is right rear, left rear, right front, left front.

As you mentioned it's best not to press the brake pedal to the floor when using the partner/pedal method. Typically you place a spacer (say, a piece of wood) under the pedal to prevent it from going to the floor. If the calipers are leaking you will see brake fluid dripping from the calipers.

[Fallinggator]

Originally Posted by Ferdman

It sounds like there is air trapped in the system. Check that the rear chamber of the master cylinder is full of brake fluid, then bleed all the brake calipers again. It's best to use a pressure bleeder. The sequence is right rear, left rear, right front, left front.

As you mentioned it's best not to press the brake pedal to the floor when using the partner/pedal method. Typically you place a spacer (say, a piece of wood) under the pedal to prevent it from going to the floor. If the calipers are leaking you will see brake fluid dripping from the calipers.

Yeah, this was my first time bleeding brakes, and I just learned the block trick. I had read about the problems with pressing too far, but I forgot to warn my partner until a little ways in to the first brake. He's not the most coordinated individual in the world though, and I'm not sure what exactly he did when he pressed the pedal at any point before or after my advice. Hopefully, just one failed attempt at bleeding 1 wheel before quitting to go buy a mityvac didn't damage the MC. I suspect with that much missing fluid...it was a LOT...that reusing the seals was a really bad idea. They looked okay, but they very well may not have seated properly or could have been damaged without me noticing it. I haven't checked for signs of a leak yet...still recuperating to be honest from all the work I did on the car..., but I will check later today or tomorrow morning. I went ahead and ordered the rebuild kits along with a new bracket for my sway bar that had cracked on general principle after reading threads, and if that doesn't fix the problem I will look into a master cylinder.

My one fear is that it has something to do with the ABS or SRS systems. Does anyone know if you can actually bench bleed the master cylinder for this vehicle without the ridiculously expensive (several thousand dollars) Mercedes special tool system? Also, I read somewhere that when working with a vehicle with ABS that you should depress the pedal 25-30 times before bleeding in order to...I want to say pressurize or depressurize the ABS system. Anyone know if that is correct?

For those who are curious, apparently when the bleeder is open...or hose detached for that matter...there is more travel in the pistons inside the master cylinder. The pistons can go further than before, and they might come across some nasty bits of corrosion or even just gunk that have formed over time in that section of the housing due to lack of use. When the seals come into contact with this section they tend to tear which causes your master cylinder to fail...thus causing increased pedal travel & spongy feeling.

I looked online & discovered two things about master cylinders. The first is that, due to liability issues, very few places sell a rebuild kit anymore. The second is that the one rebuild kit I could find was actually more than a lot of the new master cylinders I found online. Granted these were some of the cheaper brands, but if you repair the seals you might still have a badly corroded housing. Same with buying rebuilt MC's. I may be wrong, but I think I'd rather have the cheap new MC over a rebuilt one. A new good MC was not that much more than the rebuild kit either.

[Zulfiqar]

make sure you are using a catch bottle on the bleed side - this gives you the advantage of using liquid as a one way valve and it prevents air being sucked in.

raising the bottle above hub level but below master cylinder level ensures the weight of the liquid keeps the bleed submerged. Very easy to do. Bench bleeding a master cylinder is quite easy to be honest. its the same cylinder working as used by many other cars on the road.

A little experience I got was that the first 10 or so strokes on an empty system should be very small - this is to prime the system and not force bubbles to multiply into smaller bubbles. The rest is taken care of the bleed bottle setup.

[Dmitry at Pelican Parts]

Originally Posted by Fallinggator

My brake pedal is travelling way too far at the moment, and it's probably my fault...although it wasn't working before either. It started with my brake pedal being really spongy, so I decided I'd bleed my brakes. In the process I stripped out 1 bleeder valve, and had to buy a rebuilt caliper for the left rear. The other three came out eventually, and I replaced the bleeder valves to be safe. When I was examining the calipers I discovered that it was difficult to push the caliper pistons in even with a wrench, so I took them off the car to rebuild. I'm out of work at the moment, so I didn't have much money to work with. I decided to try to reuse the piston seals...probably not a great idea. I was really careful taking them off, but guessing this just wasn't going to work no matter how careful I was. On the plus side I cleaned up the pistons which weren't too bad and the piston housings, and now the pistons can move with just a bit of hand pressure. I put everything back on the car, and bled the brakes with a mityvac vacuum tool. They seemed well bled, and I took the car for a test drive. Unfortunately the pedal was going WAY too far down. It was activating the brakes, but way too much travel. I was pretty thoroughly beat after doing all this, so I just went to bed. This morning I checked the fluid level, and it was significantly below where it had been. It was right on the Max Fill Line, and now it's down probably 1/2 an inch. Am I correctly assuming that my problem is the seals are leaking, or could it also be a bad master cylinder? The first time I tried to bleed the brakes I used the partner/pedal method, and I'm worried that my partner may have pressed the pedal too far during this damaging the MC seals. Is there a way to determine this before I order the new seals for my calipers? Could there be anything else likely to cause the problem like the ABS? I've got a US 1995 mercedes E320 Wagon W124.092 M104.992.

I've attached a DIY tech article on Bleeding the Brakes on a W124, but make sure to check out the other points in the Brake section - it'll save you a lot of time and headache when addressing an issue in the future. If you need any additional assistance, please let us know and best of luck!

Bleeding Brakes (W124)


-Dmitry