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Old 09-20-2017, 11:48 PM   #1
Rusty_ratchet
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Default Low Pedal after Replacing Calipers and Bleeding Brakes

I replaced the rear calipers on my 240 and bled the brakes using the Motive power bleeder. I've done this before and have not had any issues, but this time the pedal is low. I haven't noticed it being spongy, just approx 2/3 of the way to the floor before it feels firm.

When I was replacing one of the rear calipers the fluid in the reservoir got low. I couldn't see how low because of the basket/strainer. I assume it didn't get too low because I added a little and the level was above the bottom of the basket/strainer, but I guess this could be the cause of my problems. I saw a YouTube video where a guy had air in his system, and rather than bench bleed his master cylinder he connected a tube from one of the bleeders to the MC reservoir, pushed the brake pedal repeatedly, and just kept recirculating the brake fluid. Does this seem like a reasonable approach? The pads and fluid level are all fine, are there any other possible causes for a low pedal?
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Old 09-21-2017, 07:39 AM   #2
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Air could be stuck in the rear proportioning valves and it's a royal pain to get it out. And i'd be hesitant to use the pedal for anything bleeding as it can kill these master cylinders pretty quick
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Old 09-21-2017, 08:19 AM   #3
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Are the brake bleeders pointing up on the rear calipers? It's probably air stuck in the lines, park with the rear considerably higher than the front or put in up on jack stands overnight, tap on the proportioning valves to knock any bubbles loose. I went through almost 2L of fluid before I got a good pedal last time i did it.
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Old 09-21-2017, 08:30 AM   #4
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Originally Posted by esmth View Post
Air could be stuck in the rear proportioning valves and it's a royal pain to get it out. And i'd be hesitant to use the pedal for anything bleeding as it can kill these master cylinders pretty quick
Thanks, I'll look for info on how to get air out of the proportioning valves. I've seen discussions about bleeding with the pedal ruining the master cylinder. Is it ok as long as one limits the stroke of the pedal?
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Old 09-21-2017, 08:32 AM   #5
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Originally Posted by VB242 View Post
Are the brake bleeders pointing up on the rear calipers? It's probably air stuck in the lines, park with the rear considerably higher than the front or put in up on jack stands overnight, tap on the proportioning valves to knock any bubbles loose. I went through almost 2L of fluid before I got a good pedal last time i did it.
Yes, the bleeders are pointing up and the back of the car is higher than the front. I'll look for the proportining valves and give them a tap.
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Old 09-21-2017, 08:41 AM   #6
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Quote:
Originally Posted by Rusty_ratchet View Post
Thanks, I'll look for info on how to get air out of the proportioning valves. I've seen discussions about bleeding with the pedal ruining the master cylinder. Is it ok as long as one limits the stroke of the pedal?
I would think if you limited the stroke to what it'd usually be pressed to it'd be fine but i'd get a 2nd opinion to be sure
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Old 09-21-2017, 09:17 AM   #7
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I tried using the power bleeder when installing new calipers once, what a nightmare. I couldn't get all the air out until using a the good ol' two-person pedal technique. If you're concerned about ruining your master cylinder put a block of wood under the pedal so it doesn't go too far.

After that experience I'm a firm believer that the additional pressure of the pedal helps get the air out better than the motive bleeder. Just swapping fluid? Power bleeder is fine. Trying to get air out of new components? Use the pedal, drive around for a week, use the pedal again to get the last little bits out.

Often times new calipers will have bubbles hang up on little crevices in them because they're completely dry.
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Old 09-21-2017, 08:20 PM   #8
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Originally Posted by muppetman342 View Post
I tried using the power bleeder when installing new calipers once, what a nightmare. I couldn't get all the air out until using a the good ol' two-person pedal technique. If you're concerned about ruining your master cylinder put a block of wood under the pedal so it doesn't go too far.

After that experience I'm a firm believer that the additional pressure of the pedal helps get the air out better than the motive bleeder. Just swapping fluid? Power bleeder is fine. Trying to get air out of new components? Use the pedal, drive around for a week, use the pedal again to get the last little bits out.

Often times new calipers will have bubbles hang up on little crevices in them because they're completely dry.
This afternoon I spent about an hour bleeding with the pedal. I went through the entire bleed sequence with a hose at each bleeder, feeding back to the reservoir, and making sure the level didn't get low. As I was bleeding the pedal got more and more firm. When I replaced the cap and started the car, the pedal was worse than before (no resistance at all). I put another liter through it with the Motive power bleeder. I went through the sequence several times and got a little air the first time through and no air when I finished. The pedal still feels worse than before.
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Old 09-21-2017, 10:46 PM   #9
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I'm at a loss with my brake pedal and am temped to put the old calipers back on to see if the problem is still there. What do you guys think?
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Old 09-21-2017, 11:38 PM   #10
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I would honestly point my finger at the master cylinder.
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Old 09-22-2017, 12:13 AM   #11
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I would honestly point my finger at the master cylinder.
Everything was ok before replacing the rear calipers and pressure bleeding. Why do you think the MC failed?
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Old 09-22-2017, 12:25 AM   #12
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Why did you replace the calipers? Were they frozen? I had a similar issue after replacing my frozen rear calipers where the pedal was tight and engaged right at the top beforehand. After the new calipers and them not dragging the pedal dropped what seemed to be a noticeable amount (was not really) before engaging and the pedal became a lot softer. After breaking in the brakes it is still soft but they engage nicely and can easily lock up my street tires.
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Old 09-22-2017, 07:24 AM   #13
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Originally Posted by Xeroni View Post
Why did you replace the calipers? Were they frozen? I had a similar issue after replacing my frozen rear calipers where the pedal was tight and engaged right at the top beforehand. After the new calipers and them not dragging the pedal dropped what seemed to be a noticeable amount (was not really) before engaging and the pedal became a lot softer. After breaking in the brakes it is still soft but they engage nicely and can easily lock up my street tires.
One had a stuck piston, the other one was replaced approx 10 years ago so I figured I would replace them both. After bleeding with the pedal, I don't believe the brakes engage at all.
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Old 09-22-2017, 08:57 AM   #14
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Hopefully you are using the correct sequence too bleed ?? Ran into a similar issue years back. Cured it by raising the rear about 2 feet, let it set overnight. All was ok the next day.
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Old 09-22-2017, 09:12 AM   #15
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Hopefully you are using the correct sequence too bleed ?? Ran into a similar issue years back. Cured it by raising the rear about 2 feet, let it set overnight. All was ok the next day.

I am using the bleed sequence in the Bentley manual. The back of the car is higher than the front by (guessing) 6". I'll try lowering the front a bit.
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Old 09-24-2017, 09:41 AM   #16
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Problem solved:

Based on the posts by Redwood Chair and Kjets on a Plane here, here, and here. I used the power bleeder to ensure the MC reservoir didn't run dry and vacuum bled the rear while taping on the proportioning valves and having a helper massage the pedal. I finished by bleeding according to the sequence in Bentley and massaging the pedal between calipers. The brakes are now better than they were before replacing the rear calipers.
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