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ABS fault causing intermittent sinkingbrakepedal-960

Help, Advice, Owners' Discussion and DIY Tutorials on all Volvo's "mid era" rear wheel drive Volvos.

1975 - 1993 240
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dm635
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ABS fault causing intermittent sinkingbrakepedal-960

Post by dm635 » Fri May 24, 2019 6:46 pm

I have an intermittent sinking brake pedal on my 960. Yesterday it went to the floor and drove home using the parking brake . Today felt just fine. Brake felt normal.

I ordered a master cylinder. But doing some research it's telling me it's the ABS pump since it comes and goes. Works just fine then goes soft...

My question ishow to bleed the pump or is it something else. It varies from working normal to soft. I don't feel that a master cylinder is going to fix it. Otherwise it would do it all the time.


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tardcart
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tardcart

Re: ABS fault causing intermittent sinkingbrakepedal-960

Post by tardcart » Fri May 24, 2019 7:01 pm

I think you were right to get the master cylinder, thats normally the only thing that produces a sinking pedal. However, I have never seen one go bad. also they usually do this on cold mornings, not when its hot like now. When my abs makes a pumping noise and lites up the dash, the pedal gets harder and fights my attempt to break. Im not knowledgable on this subject though. could be the earlier cars like yours vent the pressure when thinking a wheel is locked up. no light on the dash?



93Regina
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Re: ABS fault causing intermittent sinkingbrakepedal-960

Post by 93Regina » Fri May 24, 2019 11:49 pm

dm635 wrote:
Fri May 24, 2019 6:46 pm
master cylinder
I doubt ABS is the issue....

Master cylinder bleeding...I would pre-bleed it on the bench. Then take off old one, then insert new one....right away

This may not work, but I have lines loose a pinch, and place a rag around them, then have someone slowly depress pedal. When fluid starts coming out, I tighten up brake lines, while pedal is slowly being depressed. Eventually, any air will bleed out.

I've never seen a 960 brake setup...so YMMV. Otherwise, there is a technique on how to bleed 960s brake lines, which I don't have the manual to know the sequence required.



dm635
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Re: ABS fault causing intermittent sinkingbrakepedal-960

Post by dm635 » Sun May 26, 2019 8:06 am

There are no dash/warning lights. The brakes have been working as they should. Just seems to me that if it was the master cylinder it would stay soft when it acts up and not return to normal. The brakes are stiff and strong.

Not like the other night when I had to drive home about 45 miles with the parking/emergency brake. Then working fine since then.


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93Regina
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Re: ABS fault causing intermittent sinkingbrakepedal-960

Post by 93Regina » Sun May 26, 2019 1:17 pm

dm635 wrote:
Sun May 26, 2019 8:06 am
Yesterday it went to the floor and drove home using the parking brake ..... The brakes have been working as they should.
Contradiction....if needed, try pumping the brake if it fails again....penny wise, and pound foolish, if you crash.

Houston, we have a problem....



lummert
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lummert

Re: ABS fault causing intermittent sinkingbrakepedal-960

Post by lummert » Wed May 29, 2019 9:40 am

If the pedal drifts to the floor while holding steady pressure on the pedal, and you're is sitting at a light waiting for it's change, it's the master cylinder.


1988 Volvo 760 Turbo Wagon

dm635
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Re: ABS fault causing intermittent sinkingbrakepedal-960

Post by dm635 » Sun Jun 16, 2019 5:06 am

Time for an update. The brakes have been working just fine since without doing anything. I did order a master cylinder and will just hold on to it if I need it later.
When I had the problem I watched quite a few U- tube videos that did indeed say that air in the ABS pump could cause the pedal to drop to the floor. I'm leaving the braking system alone for now as it is working.


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91'745

86-635CSi 215K



Previous- 83 245T, 84 242, 85 242T, 92 960, 94 960, 95-964, 95-945

dm635
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Location: Lou. Ky.
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dm635

Re: ABS fault causing intermittent sinkingbrakepedal-960

Post by dm635 » Fri Jul 26, 2019 9:52 am

Another update. The brakes most of the time are as solid as can be. Have not replaced master cylinder. I have it on the shelf should I need it.

This is one tricky issue. My thought is that if it was the master cylinder it would be a constant ongoing problem.


96-964
91'745

86-635CSi 215K



Previous- 83 245T, 84 242, 85 242T, 92 960, 94 960, 95-964, 95-945

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