Semi-Soft Brake Pedal?!?

[Don Plumley]

Here's my problem. In prep for the Autox, I bled the brakes (see Brake Bleeding Success!). Brakes were fine during the event.

After a quick wash, I parked the car for two weeks. When I took it out yesterday, the brake pedal seemed to have more travel than I recall. I wouldn't call it spongy, just more travel. It stops fine.

No puddles under the car, MC reservoir full at the same mark. I just pulled the tires, hoses seem fine, pads have plenty of life left (about 5mm).

I'm stumped. Any reasonable guesses here?

Thanks,

Don

[makaio]

My guess, based on my own experience, is you have air in your lines my friend. I find i have to bleed my brakes after every time I go to the track.

Now I don't know what kind of breaking you were doing, but I would try bleeding them, and I bet you find some air bubbles.

[Don Plumley]

I'd like to understand the mechanism by which air gets into the lines because of a track excursion. Boiling? Bad karma?

I'll go ahead and bleed the clutch slave (didn't do it before), and while I'm at it, re-bleed the wheel calipers.

1) How much fluid do I let run out before we can say - "no air"

2) What ID tubing should I use on the clutch slave bleeder?

THANKS!

Don

[Chuck Moreland]

It is boiling of the fluid that creates "air" in the lines. Autocross will never get the brakes hot enough to do this, the courses are simply to short to create enough heat.

Actual "air" can get in the lines through a leaky fitting or bad seal.

A soft pedal can be caused by a variety of things including bad MC, bad flex brake line, caliper pistons that are retracting too far. But air in the lines is the most likely scenario and easily fixed.

It should take only a few pumps of the pedal to find any air in the lines (yes, yes, I know using the pedal is "bad for the MC". Just push the pedal half way and not to the floor and you won't have a problem). If you find bubbles keep pumping until they are gone. If they never go away, get a new MC.

Have you done anything else to the brakes recently? New pads?

[jluetjen]

Don;
What are you using for brake fluid? The PO of my car used silicon because it didn't absorb moisture while it was sitting - but I could never get the pedal solid. Due to the charactoristics of silicon it can be really tough to get all of the air out of the lines. Completely flushing the system (with Castrol DOT3) and then replacing all of the Castrol with ATE Blue solved the problem.

If you are doing a complete flush, usually a brake system will take about 1 bottle of fluid (more if the system shares the resevoir with a hydrolic clutch like in a 944). Buying the big cans only results in the leftover fluid sitting and absorbing water. If you want to be sure that you've done a complete flush, try alternating between 2 different colored fluids (ATE Blue and ATE Gold). When the fluid coming out of the cylinders changes colors, you'll know that you've flushed that line.

[Don Plumley]

I just completely flushed the system using a Motive pressure bleeder, 1st with Castrol and then with ATE Superblue. So I saw nice clean fluid fully replaced with Blue stuff.

Chuck - nothing else was done to the brakes, just a flush and bleed. Air never entered the system, so I'm stumped why it would work perfectly at the track, then two weeks later the pedal has excess travel.

I didn't think the Autox could have boiled the fluid. Why would the calipers retract too far? It could be a flex line problem, though they *look* fine. How can I tell if I need to replace the flex lines?

Thanks, Don

[Noel]

Don,

I had a similar problem with a 300ZX I owned. However, I used the old pump the brake pedal method of bleeding. It turns out it was a bad master cylinder. The theory was that I pressed the pedal father during the bleed process that is normally went and thus damaged a seal in the Master cyclinder. This is pretty common with an old mastercylinder. Did you pump the brake pedal?
When was the last time your MC was replaced?

I would start with the simple less expensive things first. Like bleeding and bleeding. Try also tapping your calipers with a wrench while bleeding, sometimes this will release trapped air.

Good luck,

Noel

[Don Plumley]

Noel -

I didn't pump the pedal - I used a Motive pressure bleeder. The MC looks to be original, but the car is only an 87 with 66K miles. Hard to believe the MC, or even the soft lines are the problem.

My dilemma is just as you described. I *should* replace the front struts, soft lines, maybe replace/repack the bearings, plus an argument for replacing the pads and rotors. Doing any one item gets, in theory, progressively easier (or at least less duplicative work in the future) when you do the whole kit and kaboodle.

(see my related thread: Front bearings: Repack, adjust, do nothing?)

It's just a bad month to drop over $500 on the front end (mostly guilt on my part) when, aside from the soft pedal, stuff works fine.

I'm really stumped - part of me wants to just do a quick bleed and see what that does, part of me says to go ahead and replace the soft lines (which means do the bearings...).

Thanks for listening guys. Any comments/feedback/advice appreciated.

Don

[Rick Lee]

Hmmm - this touches on an issue I'm having too. I flushed my brakes recently, did the pedal method and probably pushed it too far. I think I pushed it to the floor a few times. I later learned of the problems that causes the MC seal. Then my clutch slave cylinder went bad and I'm still dealing with that. My brake pedal feels and performs fine. Is there a chance that my brake flush did not screw up my MC seal? And if it did, could it have caused any of my clutch slave cylinder problems too? When I removed the clutch slave, it was leaky. But I thought it too coincidental, since it happened a week after I flushed the brakes. Am I in for a new brake MC soon? Mine is also an '87 Carrera, with 81k miles.

[Don Plumley]

Noel/Chuck - you were right - there was air in the lines.

I just finished a re-bleed and there were a few bubbles in the line. I used this opportunity to bleed the clutch slave cylinder too.

As I finished up, I needed to get some extra fluid out of the MC. So I had my wife do a little brake pedal bleeding. On the last wheel, she said she pushed the pedal down pretty far once. Crossing my fingers and hoping that didn't do anything bad to the MC.

I'll autox the car again later this month, will be interesting if this happens again...

Thanks for the advice gang, could not have done it without your support.

Don

[makaio]

Don,

Glad to hear you have your brakes fixed!

I told you it was air!

[Don Plumley]

Yeah Matt - you were right as well - Thanks!

DJP

[KTL]

Don,

Next time you bleed your brakes, couldn't hurt to check the brake light switches on the MC. When I was looking around that area I decided to pull the elec. connectors off the switches. Curious since my brake lights come on a little later than i'd like. Turns out one of the switches was oh so slightly leaky. Just moist and blue. No drips. Regardless of the degree of leakage, no leakage of the brake system is acceptable.

May not be related to your problem, but just thought i'd mention it.