Bad Master cylinder?

[LEAKYSEALS951]

Couple of questions for the braintrust. Bonus points if you can tie it together into one problem.

Problem #1. I'm wondering if this is a MC, Brake Vacuum, or air in the brake line problem

Volvo brakes (2005 xc70) work- Brakes engage in top 1/3 of brake pedal stroke. Occasionally, no brake engagement in top 1/3- brake pedal unnervingly drops to lower 1/3 of stroke before engaging, at which point it works. (phewwwwww!!!!)

1.Regardless of engagement point, once it engages, it works. The pedal does not "slowly sink" afterwards, like when bleeding brakes.
2. No loss of brake fluid. Fluid relatively changed and ok.
3. If the pedal drops to lower 1/3, quickly releasing and rebraking lets the MC recharge and the pedal engages in upper 1/3 as normal.
4. No sponginess as far as I can tell.

Problem #2. ABS issue.
The code thrown is intermittent- low ABS signal. From what little I can garner, this indicates either a bad ABS pump, or a crapping out ABS computer, the computer especially gets a little complex to fix (remove unit and send to xemodex for reprogramming to new computer.) I do think I am getting 12v to the computer.

1.This goes off, but the ABS system still works. last year I went out and purposely flogged/ engaged/ duke boys the system in snow/ice to try to see what was wrong, but it seems to work fine.

If you can tie these together so addressing the brake issue- say new MC or Vacuum leak fixes the ABS codes (so I don't have to mess with that), I will buy you a pizza or something!

Thanks!
R

[john70t]

My first guess was air in a lower line. Maybe from a leaking hose introducing air. But that would be constant.

But probably caliper seal + fluid contaminant.
You apply full pressure and it seals itself back up usually. Retraction introduces air.
Thus the random part.

[908/930]

I'm thinking your pistons in the master cylinder are not returning up, possibly corrosion or broken return spring in cylinder.

[fastfredracing]

I wonder if you are getting false ABS activation , from a bad, or slightly skewed wheel speed sensor . When the abs activates, the pedal will either drop, or sometimes become incredibly firm . Get me the ABS code#, and we can probably dive deeper into this . I bet they are related .

[LEAKYSEALS951]

Originally Posted by fastfredracing

I wonder if you are getting false ABS activation , from a bad, or slightly skewed wheel speed sensor . When the abs activates, the pedal will either drop, or sometimes become incredibly firm . Get me the ABS code#, and we can probably dive deeper into this . I bet they are related .

Thanks all.
Heading out to dinner, and my code reader is not with me at the moment I think it was ABS / BCM-0070. It's a rather generic code from mid level reader, not a VIDA-DICE level info. I will confirm tomorrow.

I've had wheel sensor codes in the past- infact, that's what I was originally hoping, but this is different code.

Also-for more info- there doesn't seem to be correlation between the brake pedal diving and the abs light. The brake pedal happens rarely, maybe 1 in 100 pedal strokes. The ABS light is off about 75% of the time. The code trips (randomly), but after a couple of miles it resets itself to off. No change in braking behavior when on- like last year- I was gunning it in the snow and it worked okay. It also seems to trip less in cold weather (which could be an illusion), which was making me hope it was power supply related to the unit. Earlier I stated it was getting 12v, but I haven't really done that. I cleaned the connections, but need to isolate the actual power feed and confirm power.

My gut hope (if they are related) is that an old school brake issue was perhaps occasionally tripping the newer school abs system, but it's more a hope based on me not wanting to mess with the ABS.

Disconnecting the ABS control module involves a bunch of brake lines which looks like it would be a challenge to bleed.

[LEAKYSEALS951]

Originally Posted by 908/930

I'm thinking your pistons in the master cylinder are not returning up, possibly corrosion or broken return spring in cylinder.

The piston not returning in the master cylinder makes a lot of sense. I wouldn't be surprised. The only MC failure I've had the actual piston was leaking, so there was a slow settling of the pedal.

Your idea would explain why there is no fluid loss/pedal settling. The ABS code might just be a different beast.

[rfuerst911sc]

What does fluid relatively changed and ok mean ? If that means recently bled any chance air was introduced ?

[LEAKYSEALS951]

Originally Posted by rfuerst911sc

What does fluid relatively changed and ok mean ? If that means recently bled any chance air was introduced ?

Good question. Brake fluid not recently changed- but overall maintained. I think I did it about 2-3 years ago, but I didn't muck it up (and I am good at that! )

The last change didn't correspond with the issues. The issues are newer.

It's currently full and could stand a refresh, but not horrific in color.


At this point I'm thinking testing voltage to the unit, new MC, potentially talking to xemodex about the ABS unit.

My biggest concern is being able to purge the ABS unit of air if I replace the MC or the ABS unit. I think I remember some horror story here about someone who got air in the ABS unit and couldn't get a good bleed on it. Something about driving down a dirt road to shake out the bubbles or something.

There's good access to both components, so that's not an issue, just the purging. I did notice tonight, a rat has been gnawing at the brake fluid reservoir, so I'm going to inspect wiring tomorrow for damage, although I did not see any the last time I looked (when I replaced the head earlier this summer).

This thread has already helped in that it gives me confidence the MC could be at fault, not a brake booster vacuum issue. I've been working on a strategy for this for a while now. Time to go do something.

[porsche tech]

I’m with Fred…maybe a wheel speed sensor. When you hit the brakes and the pedal sinks if you let pressure off the steering wheel it may pull to one side or the other indicating a front wheel speed sensor.

[LEAKYSEALS951]

Originally Posted by porsche tech

I’m with Fred…maybe a wheel speed sensor. When you hit the brakes and the pedal sinks if you let pressure off the steering wheel it may pull to one side or the other indicating a front wheel speed sensor.

Ok- I'll check that out. (you guys are the experts) But... shouldn't that throw a sensor code? If I remember it would be something like "Left rear speed sensor" or something like that. I've had speed sensors go bad on this car in the past, sometimes even by my own bungling "when I'm in there" replacing struts or something (don't tell anyone that) but the car always seems to tell me where the problem is (in terms of the speed sensors).

The ABS code does pop up though like a speed sensor code... but thinking about it, the speed sensor codes ABS light generally stick. This ABS light erases itself until it screws up again. No reset of codes needed. It's like a glitch in the matrix, not a broken wire that some dumbarse broke while he was in there!

Seeing the rat/mouse damage really gives me more leads to go look at tomorrow in the daylight. This car isn't soy based, thank god, but the brake reservoir sure kept the rat's teeth sharp.