1996 850 GLT, 110K

When I am sitting at a traffic light, my brake pedal sinks slightly, little by little. Never has gotten close to the floor, and no dash lights have illuminated. I'm wondering if this might be caused by a bad master cylinder or gas/air that got into the brake fluid. Doesn't the master cylinder actually contain two circuits, and wouldn't I get a light if the cylinder failed? Is the best way to find out to re-bleed the brakes (which were done when I changed my pads and rotors about 6 months ago)?

ok try this test....

drive the car, park up put foot on brake, put car in park, put handbrake on turn engine OFF....

now pump the brake pedel several (maybe up to 20) times unitl it goes hard (you will know when it does!)...it should get to a point where it moves a bit then its like hitting a brick wall!


ok if it then sinks away from you under presure you have a couple of possibles in my book..

1. faield seals in the master cylinder allowing the fluid to flow back into the reservior...

2. a leak some where in the system....are you using brake fluid - check you reservoir...mark it with a pen so you can keep a CLOSE check..

if you have air i nthe system the pedal will go to feel 'spongy' sort of like a 'bounce' in it....but should not just continue sinking....though IF you havea leak in the system then you probably will have air in the system as well.

thanks--i'll try this. by "park up," you mean on a hill, or just put it in park?

Just put it in park. If the brake fades, let your foot go all the way to the floor, then do it again. Turn off the car and look for brake fluid on the inside of all four wheels. Also look under the MC. It looks like heavy water.

Klaus
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(V♂LV♂s 1975 164, 1995 854T, 1998 V70R)

well i put it in park and pressed on the pedal as suggested, released, repeated... the pedal does firm up quite a bit, but if I give it enough pressure, it sinks. no apparent fluid leaks, so must be faulty/failing seal between pistons in the MC--it's an original unit, so maybe no surprise. wouldn't this mean that braking power is coming mostly/only from one set of wheels?

Paul,
My 1st MC did just like yours, but there was enough pressure in both lines to make it work. The 2nd MC had a really bad seal and with heavy pressure you realized that only line was working. Really scarey driving on city streets.
Tell the shop to replace the seals and you will be happy.

Klaus
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(V♂LV♂s 1975 164, 1995 854T, 1998 V70R)

I don't think that air could newly enter into the system unless it was leaking or the fluid otherwise ran so low that the reservoir was empty. There is a warning light for low fluid in the reservoir. But if the system was opened then not filled properly in the past, some historical air could be trapped. It does not take much air to cause a soft pedal, just a few milliliters.

The experience of Klaus fits your situation if the brakes were working and the soft pedal became noticeable later. If the soft pedal was always present and there's been no loss of braking performance - some drivers who had a disk/drum setup and go to a 4-disk setup report a softer pedal which is normal.

The 2 halves of the master cylinder are only independent as long as the seal between them holds.

Regarding repeated failures as described by Rancher Rick, the question comes up as to how the brakes were bled - whether aggressively hand bleeding (by repeatedly stepping fully on the pedal to push fluid through) may have weakened or damaged seals in a new master cylinder. It seems to me that a vacuum method which does not involve pumping the pedal is safest.

Hi,
My wife just had the MC replaced in her 95 wagon, She had the pads/rotors done several months ago, the lines were bled but there was still considerable(way too much) pedal travel before you could get any braking response. So we had the MC replaced(OEM), went thru the process again & the brakes would be good for a week & then same problem again. Scary pedal travel. Replaced the MC under warranty & the whole scenario repeats itself. All together these brake lines have been bled at least 4 times & it 1s getting frustrating. I trust our local indie shop where we get all our servicing done(truck & Volvo) but this isn`t rocket science.
Any ideas/solutions?
Thanks,
Rich

Hey Rancher,
After my second MC, I asked my indie if he would just "rebuild" the last one instead of trying another. That worked. Seems the shelf life of the seals is only so long.

Klaus
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(V♂LV♂s 1975 164, 1995 854T, 1998 V70R)

Sounds like one of the seals inside the master cylinder has failed. A rebuild kit is available for that, or a new master cylinder.
My 164 just had that same problem, very troublesome when you have to shift to neutral at a traffic light so you don't creep up on the other guy's bumper.

Klaus
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(V♂LV♂s 1975 164, 1995 854T, 1998 V70R)