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Not a pathy but soliciting ideas


k9sar
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My 79 year old mother's Cadillac SRX (part of my inheritance..eventually) suddenly lost brakes.  She came out of a store and when she hit the pedal, it went to the floor.  Brakes had been working fine up until then.  About a year and a half ago, all the hard lines had been replaced due to corrosion and her pads were replaced about 6 months ago.  Anyway, she was able to limp it to her house.  I stopped over to take a look and found the following:

Running or not, the brake pedal goes to the floor when pressed.

The brake fluid reservoir is full.

There are no observed leaks around the master cylinder or junction block for the rear lines.

After pressing the brakes a dozen or so times to the floor, there was no brake fluid leaked onto the driveway or any leakage around the calipers.

So immediately I thought... master cylinder failed and ordered a replacement.

Removed the hard lines (2) from the master cylinder and capped/plugged them.

Swapped the reservoir to the new master cylinder and filled it with fluid.

plugged the ports on the MC and held it in a bench vice.

Using an old push-rod from a blown dodge engine, I forced the piston in the MC until it was no longer burping air and the piston was unable to be depressed.  I felt that was a successful bench-bleed so I installed it in the vehicle, including using a syringe to add fluid to the rigid lines before installing in the MC (barely took any so I'm thinking that no air should have gotten into the system other than maybe a tiny bubble.  I was very careful not to introduce air since it was cold and pouring down rain and I didn't feel like crawling under and bleeding the system.

Got everything tight and wiped down... got in the vehicle and tested the brakes..... right to the floor  DAMNIT!

so, now I'm at a loss as to what the problem could be.  What could cause a sudden loss of brakes that did not involve a ruptured line or failed MC?  My understanding of the vacuum assist is that there is a hard linkage there through the diaphragm so even if the VA failed, brakes would still work. 

Any thoughts on where to look next?

oh yea, and some clown on the Cadillac forums tried to tell me that a loose hub or wheel axle would cause that since the piston on the caliper would have to push much further if it were loose.  He obviously doesn't understand disc brakes.  :D

 

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The suggestion of bad hubs isn't entirely off base from what I've read (pistons get pushed back by the rotor wobbling, so you end up with free play in the pedal extending the pistons back out until the pads meet the rotors) but you'd only have the problem while the car was moving, and then it would pump back up after a few strokes.

 

The Pathy brake booster I had apart did not have a solid rod from input to output, but the two rods met in the middle and should give a solid pedal whether the diaphragm is working or not. The slop between the two rods is what opens and closes the valve that controls air pressure against the back of the diaphragm in the booster. But no, unless the booster went entirely to pieces inside, a failed booster should not allow the pedal to go to the floor. (I replaced mine thinking it was the only thing left that could be wrong, and it solved nothing.) I'd be hesitant to say the new MC didn't do it without bleeding the thing first, even though it sounds like you were careful with it. I'd also be suspicious of the ABS system.

After the fun I had last summer with mine, I hate brakes. There's no telling what's wrong unless it's leaking, or where the air is, or if there's air, or whether the new part or the bleed was what changed something.

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