Brakes: now you see 'em, now you don't!

[jtosburn]

Long time lurker emerging with a question:

Last week I was out driving my 151k mile '78 SC, when, after thirty minutes or so of driving, I went for the brake pedal and found my foot on the floor. Yikes! I got some brake action by pumping the pedal, and proceeded to drive gingerly home, noting that always the first push of the pedal produced a very small bit of feel at the last inch or so of travel, and more feel on the next pump, but still very soft. I had noted earlier in the month that after longer drives, when pulling into my driveway, there was a humming sound from the driver-side front, and have wondered if a caliper piston were sticking; this may or may not be related and is offered for context.

The brakes had not been bled in over three years, and hoses seem original. I had been meaning to bleed and replace the soft lines for a while, so that time is now. Since I could see no evidence of leaking brake fluid anywhere and the level of fluid in the reservoir was ok, I also procured a new master cylinder from our host. That seemed like the obvious failure point.

Saturday, with the help of fellow Pelican Scott Clarke, we installed the new master cylinder (no evidence of current leaks, but old rusty crawdoo present), and the new soft lines at the rear. The soft lines at the front were frozen solid to the hard lines going to the calipers, so I will be ordering those, and replacing both next weekend. Bled brakes with new ATE Super Blue. Looked good. But...

After driving around town for a bit, and increasing confidence, I took the car on a short, higher speed jaunt, still allowing plenty of space for braking. On my way back into town, pedal to the floor again! The symptoms were the same in that the last bit of travel had a little feel, and the next pump produced enough stopping effort to be of use. And that noise from the left front at the end was louder, and audible for several blocks before home.

So what's going on here? Are thirty year old front soft lines enough to cause this without any visible leakage? Could a single dragging caliper piston cause enough heat to produce that much fade? What other failures do I need to look at?

Pads are ok, rotors don't seem damaged but haven't measured runout or anything. Vacuum assist seems to function fine, but even if it weren't, I'd just have greater effort, not necessarily pedal travel, right?

[john walker's workshop]

possibly a caliper piston is stuck, causing one or both pads to drag on the rotor. the heat transfers through the piston to the fluid and the fluid vaporizes. cools down and you have pedal again. be sure the pedal push rod has a bit of freeplay in it so the master cylinder piston can fully return.

[jtosburn]

Thanks for the reply, John.

The pedal does have a bit of free play. And pressure does seem to return after the car cools down, so that jibes. It looks like rebuilt calipers are in my immediate future, unless there are any other theories.

[john walker's workshop]

usually you would have one or more wheels that are hard to turn by hand.

[bigsid]

Been there done that .... I agree with mr Walker. Feel your hubs when you get to a stop.... one will probably be so hot you can't touch it.
John

[Walt Fricke]

JT

After you pump the brakes for stop (or slow) #1 after this happens, do you have to pump as much or the same for your next application of the brakes (allowing for the fact that you are now driving so as not to have to use brakes)?

If something causes the pads to be pushed back farther than normal (which is not much at all), you will get a spongy pedal. Extreme example is after changing pads - if you forget to pump the brakes you get surprised when first you use them again. All due to too much travel, not boiled fluid with the compressible air (or is it fluid vapor?) involved.

I particularly realized this, in a less dramatic way, due to the wear pattern on a set of rear rotors. A lip built up on the outer circumference. Pads a bit short, didn't wear here I guess. But sometimes this lip would still push the pads out a bit more from the now recessed wear surface. Given the tolerances and mechanical/hydraulic force multiplications, having a pad just a little bit farther out from the rotor can cause what seems like a whole lot of extra pedal travel.

Distorted rotors do this in spades. But here the differential diagnosis gets shaky, because you usually feel a warped rotor through pulsations in the pedal, no?

One nice thing about having an infrared thermometer gun is you can easily check rotor temperatures. If one is a lot hotter than the other at a given end, that's probably the culprit, be it due to dragging or some other cause.

Speaking of which, the common wisdom about old rubber lines is that they can delaminate, with a flap inside forming a one way valve. Which prevents the pistons from retracting as they should. Causing drag = heat. And it is the fronts you couldn't replace due to the frozen fittings (a very common ailment). Come to think of it, doesn't this fit your symptoms, including the fact that everything but front lines (and calipers, of course) has been gone over? New fluid, new rear lines, and something that tells you it is the fronts, not a rear?

Walt

[RWebb]

Hi Joel,

I'll be interested to hear what you find re one hub being hot...

I agree that you should change all your rubber lines regardless - and do it every 15 years or so.

Best, - Randy

[jtosburn]

Well, the left front wheel is significantly more difficult to turn than the right, so the dragging piston theory is looking pretty good. Whether the old, soft rubber lines are to blame, or just plain old age, rebuilt calipers seem to be the next step. Replacing the front rubber lines and hard lines to the calipers is already a given.

Walt, once the problem manifests, the pedal effort is roughly the same until the brakes cool. I may try to borrow an infrared thermometer and do a temperature test, but truth be told, I don't really relish driving the car in its current condition.

Thanks for the replies!

[James Brown]

When I do brakes I do it all at the same time-rotors, bearings, break lines, hoses, calapers, master cylinder, pads, new fluid. that way there is no "old problem" lurking in the system. Unless I screw something up! then it's a whole new problem!!! Good luck