Internally leaking master cylinder?

[Sukhoi]

I am under the assumption that if my master cylinder were to be leaking internally the pressure differential between the two pistons should cause the brake warning light on my dash to illuminate? Is this correct? I know the light works because several years ago I had a bleeder plug wiggle open and the light alerted me of it.

The issue is my brake pedal is very spongy. If I leave a constant pressure on the pedal it does not sink further. From what I understand this points to air in the lines, not an internally leaking master cylinder. But I had a full flush of the brake system last summer and I had the same sponginess before and after the flush so I would be surprised there is still air in the system.

 

[Zenmervolt]

The light only indicates if the fluid level is low.

Air in the lines is possible even after a flush and fill if the ABS system was not also flushed properly. Also, some cars just have a spongy brake pedal.

ZV

 

[Sukhoi]

Originally posted by: Zenmervolt
The light only indicates if the fluid level is low.

Air in the lines is possible even after a flush and fill if the ABS system was not also flushed properly. Also, some cars just have a spongy brake pedal.

ZV

Oh, no ABS. I guess I'll start with another flush and if that doesn't work then replace the cylinder. It's way too spongy to be normal...friends that drive my car get scared.

 

[Zenmervolt]

What kind of car?

ZV

 

[Sukhoi]

Oh, sorry. 1995 Corolla w/ 1.6L.

I'm pretty sure when I had that leaky bleeder valve a while ago that the light did indeed tell me there was a pressure differential in the master cylinder. I remember filling up the reservoir, and the light going on immediately after I hit the brake pedal even when the reservoir was still mostly full.

 

[Zenmervolt]

Originally posted by: Sukhoi
Oh, sorry. 1995 Corolla w/ 1.6L.

I'm pretty sure when I had that leaky bleeder valve a while ago that the light did indeed tell me there was a pressure differential in the master cylinder. I remember filling up the reservoir, and the light going on immediately after I hit the brake pedal even when the reservoir was still mostly full.

The light is triggered by a simple float mechanism in the reservoir. "Mostly full" is often sufficiently low as to cause the float to contact under braking or during cornering. That light isn't going to tell you whether or not there's a pressure differential.

ZV

 

[thedarkwolf]

None of my cars have had a simple float mechanism in the reservoir. Even my 67 ford galaxie had a pressure differential switch. http://www.mattsauto.com/b1.htm. I've only had the light come on once when my master cylinder was dying, had it happen a bunch, and on that one I stepped on the brakes and nothing happened at all other then the light coming on. Never had any warning and they just went right to the floor. Pumping them got them working again until the next time it happened a few miles later. When my cylinder was just leaking internally it wasn't enough to kick the light on.

 

[Zenmervolt]

Originally posted by: thedarkwolf
None of my cars have had a simple float mechanism in the reservoir. Even my 67 ford galaxie had a pressure differential switch. http://www.mattsauto.com/b1.htm. I've only had the light come on once when my master cylinder was dying, had it happen a bunch, and on that one I stepped on the brakes and nothing happened at all other then the light coming on. Never had any warning and they just went right to the floor. Pumping them got them working again until the next time it happened a few miles later. When my cylinder was just leaking internally it wasn't enough to kick the light on.

Huh. I've never seen that before. On old german cars it's always a simple float. Hell, even on my '96 Mark VIII it was a simple fluid level float sensor. Learn something new every day. This is what I get for wrenching on old cars with questionable histories.

EDIT: Upon reading further, the pressure differential switch does not monitor the master cylinder. Rather, it monitors the two (or more) circuits of a multiple circuit braking system. It won't show a leaking master cylinder because that would not cause an imbalance in pressure between the multiple brake circuits, it would simply cause low (but balanced) pressure in all circuits.

The sensor in the reservoir remains a simple float sensor for fluid level, I was thinking this was the sensor he was talking about since he mentioned the master cylinder.

ZV

 

[Sukhoi]

Originally posted by: Zenmervolt

Originally posted by: thedarkwolf
None of my cars have had a simple float mechanism in the reservoir. Even my 67 ford galaxie had a pressure differential switch. http://www.mattsauto.com/b1.htm. I've only had the light come on once when my master cylinder was dying, had it happen a bunch, and on that one I stepped on the brakes and nothing happened at all other then the light coming on. Never had any warning and they just went right to the floor. Pumping them got them working again until the next time it happened a few miles later. When my cylinder was just leaking internally it wasn't enough to kick the light on.

Huh. I've never seen that before. On old german cars it's always a simple float. Hell, even on my '96 Mark VIII it was a simple fluid level float sensor. Learn something new every day I guess.

EDIT: Upon reading further, the pressure differential switch does not monitor the master cylinder. Rather, it monitors the two (or more) circuits of a multiple circuit braking system. It won't show a leaking master cylinder because that would not cause an imbalance in pressure between the multiple brake circuits, it would simply cause low (but balanced) pressure in all circuits.

The sensor in the reservoir remains a simple float sensor for fluid level, I was thinking this was the sensor he was talking about since he mentioned the master cylinder.

ZV

Ah, that makes sense. I figured there was just a sensor on each side of the master cylinder, not down in the lines somewhere.

If my master cylinder were leaking internally the pedal would have to slowly sink to the floor unless all the fluid was blowing by immediately after pushing the pedal, right? I definitely don't get sinking...

 

[Zenmervolt]

Originally posted by: Sukhoi
Ah, that makes sense. I figured there was just a sensor on each side of the master cylinder, not down in the lines somewhere.

If my master cylinder were leaking internally the pedal would have to slowly sink to the floor unless all the fluid was blowing by immediately after pushing the pedal, right? I definitely don't get sinking...

Thinking about it (which is dangerous for me), the sensor is probably physically attached to the master cylinder since that's where the lines for multiple circuits will originate, but it probably just bridges the two lines rather than monitoring the master cylinder too. Of course, I could well be wrong, I only just discovered this pressure differential switch today, after all.

If it were leaking internally, it should sink slowly, yes.

Try this:

With the car off (engine not running, key out of the ignition), press the brake pedal several times. The brakes should become noticeably firmer (since you'll use up any residual vacuum assist in the booster). If the pedal firms up (it should feel almost like you can't press it at all, not quite that bad, but very, very hard to press), then it's possible that the issue is with the brake booster and not with the master cylinder at all.

ZV

 

[Sukhoi]

Originally posted by: Zenmervolt

Originally posted by: Sukhoi
Ah, that makes sense. I figured there was just a sensor on each side of the master cylinder, not down in the lines somewhere.

If my master cylinder were leaking internally the pedal would have to slowly sink to the floor unless all the fluid was blowing by immediately after pushing the pedal, right? I definitely don't get sinking...

Thinking about it (which is dangerous for me), the sensor is probably physically attached to the master cylinder since that's where the lines for multiple circuits will originate, but it probably just bridges the two lines rather than monitoring the master cylinder too. Of course, I could well be wrong, I only just discovered this pressure differential switch today, after all.

If it were leaking internally, it should sink slowly, yes.

Try this:

With the car off (engine not running, key out of the ignition), press the brake pedal several times. The brakes should become noticeably firmer (since you'll use up any residual vacuum assist in the booster). If the pedal firms up (it should feel almost like you can't press it at all, not quite that bad, but very, very hard to press), then it's possible that the issue is with the brake booster and not with the master cylinder at all.

ZV

Now I'm confused. If it firms up like it's supposed to with the car off, why would that mean there's a booster problem? Or are you saying if the master cylinder is bad I should get sinking even with the car off?

 

[EightySix Four]

Originally posted by: Sukhoi

Originally posted by: Zenmervolt

Originally posted by: Sukhoi
Ah, that makes sense. I figured there was just a sensor on each side of the master cylinder, not down in the lines somewhere.

If my master cylinder were leaking internally the pedal would have to slowly sink to the floor unless all the fluid was blowing by immediately after pushing the pedal, right? I definitely don't get sinking...

Thinking about it (which is dangerous for me), the sensor is probably physically attached to the master cylinder since that's where the lines for multiple circuits will originate, but it probably just bridges the two lines rather than monitoring the master cylinder too. Of course, I could well be wrong, I only just discovered this pressure differential switch today, after all.

If it were leaking internally, it should sink slowly, yes.

Try this:

With the car off (engine not running, key out of the ignition), press the brake pedal several times. The brakes should become noticeably firmer (since you'll use up any residual vacuum assist in the booster). If the pedal firms up (it should feel almost like you can't press it at all, not quite that bad, but very, very hard to press), then it's possible that the issue is with the brake booster and not with the master cylinder at all.

ZV

Now I'm confused. If it firms up like it's supposed to with the car off, why would that mean there's a booster problem? Or are you saying if the master cylinder is bad I should get sinking even with the car off?

He is saying that if the master cylinder is bad the pedal will sink even when the cars off and the vac is gone.

 

[Zenmervolt]

Originally posted by: Sukhoi

Originally posted by: Zenmervolt

Originally posted by: Sukhoi
Ah, that makes sense. I figured there was just a sensor on each side of the master cylinder, not down in the lines somewhere.

If my master cylinder were leaking internally the pedal would have to slowly sink to the floor unless all the fluid was blowing by immediately after pushing the pedal, right? I definitely don't get sinking...

Thinking about it (which is dangerous for me), the sensor is probably physically attached to the master cylinder since that's where the lines for multiple circuits will originate, but it probably just bridges the two lines rather than monitoring the master cylinder too. Of course, I could well be wrong, I only just discovered this pressure differential switch today, after all.

If it were leaking internally, it should sink slowly, yes.

Try this:

With the car off (engine not running, key out of the ignition), press the brake pedal several times. The brakes should become noticeably firmer (since you'll use up any residual vacuum assist in the booster). If the pedal firms up (it should feel almost like you can't press it at all, not quite that bad, but very, very hard to press), then it's possible that the issue is with the brake booster and not with the master cylinder at all.

ZV

Now I'm confused. If it firms up like it's supposed to with the car off, why would that mean there's a booster problem? Or are you saying if the master cylinder is bad I should get sinking even with the car off?

A booster with problems can cause poor pedal feel. If the brakes are working as they should but the pedal just feels spongy, the issue may lie in the booster (it's out there, but it's happened). Also, if the pedal firms up and doesn't sink with the booster off, the master cylinder probably isn't leaking internally.

ZV

 

[Sukhoi]

I tried it out with the car off, and no sink. It firms up some but not as much as it probably should...still is spongy. At least it seems it isn't the master cylinder. The booster shouldn't be doing anything with the car off so the fact it's still spongy means air in the lines, right?