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Hi all,
Completed the brake rebuild on my son's '83 245: new master cylinder, rotors, calipers (switched from solid to vented rotors), and pads. Bench bled the master cylinder, then bled the system. All went as expected. Good pedal, solid as a rock.
However, when we start the car, the pedal goes completely soft. Is this the brake booster? I thought that, if the booster failed, you still had manual brakes.
Neither Bentley or FAQ is helping me out.
Thx,
Monty
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Hi Monty;
The classic problem that may arise after a brake rebuild on our cars, is that the calipers are not installed correctly left/right. If this is the situation, you will never be able to get all the air out and the pedal will be soft. A variation of this problem is that when buying reconditioned calipers, the two halves may be mismatched.
Start by checking your calipers and make sure that there is a bleed nipple on top pointing inwards, and not one at the bottom pointing outwards. This is how it should look (front):
Erling.
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My 240 Page
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. . . and once you have established which end is up, look on the bottom of the caliper body castings. There should be two match marks in the form of "divots" in the castings of the caliper halves, opposite of each other. At times, these markings can be a little indistinct, but once you know where to look they are readily spotted.
If the caliper halves are not matched properly, the the outer, bottom cylinder becomes a dead end which is absolutely impossible to bleed.
One time, before the good people of this forum informed me that it was not unheard of for rebuilders to assemble calipers incorrectly, I spent a week from hell trying to isolate this problem.
Rich
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I don't think that's it, Girling. We've got solid pedal. We pushed the car down the street with the engine off, and the brakes work as they should. Start the car and they're gone. I'm flummoxed.
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It's normal for the pedal to sink about 1/2" when the engine starts and the booster gets vacuum. I can't explain why but know it happens (see Bentley 520-5, Booster Quick Check). There's a lot more system pressure than you can apply at the pedal.
I think the problem is in the bleeding. Give us some details.
When everything is new, I think it helps to apply full pedal pressure occasionally during each piston's bleed cycle to take up all the rotor-pad-grease-shim-piston clearances. If you don't do it, the booster will — accumulating a bit more pedal travel at each of the 6 pistons.
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Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.
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We'll give this a shot. We were trying to avoid pressing the pedal too far, as apparently that can damage the master cylinder.
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So you're pedal bleeding? Nothing wrong with that, IMO. Especially with a new MC. It's the thought of seal damage from potential rust at the end of an old MC bore that worries folks. A piece if 2x4 under the pedal prevents that.
Are you also saying you couldn't ever feel any pedal pressure while bleeding? If so, that might be due to the sequence error. You may have been pushing air around but never completely bleeding one triangular circuit before moving to the other, then doing the same.
My suggestion to lay on some pedal assumed you'd feel some resistance after going around both "triangles" at least once. But I guess that never really happened. See what happens tonight.
With all you've replaced, I'd recommend going thru the correct sequence at least twice.
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Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.
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"Are you also saying you couldn't ever feel any pedal pressure while bleeding?"
No, we felt good pressure as we went along, and by the end, had great pressure. With all bleeders closed, you could not push the pedal past a certain point.
So: pushing the pedal too far is more of a problem with an existing (i.e. older) master cylinder, and less of an issue with a new one? That might be our problem. We were being very careful not to depress the pedal too far (which is contrary to how I've always bled brakes).
I also may pull the master cylinder back off and adjust the push rod out a bit. We may have a bit too much travel at the top of the pedal.
As always, thanks for all your replies. It's invaluable.
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"We were being very careful not to depress the pedal too far (which is contrary to how I've always bled brakes)."
I agree, and wouldn't worry about full pedal travel on a new MC.
But I wouldn't tinker with the booster rod unless you need to "un-tinker".
It's not something I ever had to touch.
--
Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.
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I went back to your earlier post and took a second look at this:
"Bentley's bleed order is: LR, LF top, RR, RF top, LF two bottom bleeders, RF two bottom bleeders."
I failed to notice it on first reading, but you either read it wrong or mis-typed the above. My version of Bentley's sequence to highlight the two separate hydraulic circuits would be:
Circuit 1: LR - LF Top - RF Top
Circuit 2: RR - RF Lowers(2) - LF Lowers(2)
--
Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.
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Actually.... you are correct Lucid. We did the bleed out of order. We'll go back tonight and re-bleed it. I wouldn't have thought that the order would have been so critical. As I said earlier, it felt like we were getting good pedal, but...
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As I just said, montysano, check the match marks. I had similar symptoms when I installed an incorrectly assembled caliper.
Rich
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I'm truly an idiot. I read the warning about the match marks, and took a quick look, but not a good look. On closer inspection, my new RF caliper has mismatched dimples! Gahhh!!! New caliper on the way. I'll report back later.
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...you can prevent losing any fluid (or letting air in) if you follow the tip I put in THIS post .
Of course you'll have to bleed the new caliper but the rest of the system should be "good" (or at lest unchanged).
Note that the mis-match only affects the lower outer cylinder, thus preventing a good bleed on the RR - RF(lower) and LF(lower) triangle circuit.
Although the RF(upper) on your new caliper IS in the other triangle circuit, the tip I linked will keep all but a drop or two of fluid from leaving that pipe when opened (at the upper fitting).
--
Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.
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