front brakes dragging

[Coolbreeze]

Shortly after replacing the power brake booster on my ’80 Fiat Spider 2000, the right front brake started dragging after driving ~20 miles. The problem was so severe that it overheated the brake and boiled the brake fluid out of the system the first time it happened. After that, the front brake(s) would start dragging after about 4 miles. Although the right front brake caused the most trouble; to a much lesser extent, the left front brake would also drag.

Three different mechanics tried to correct this problem, with no success. They adjusted the begeebers out of the new booster and replaced the front calipers, brake lines, rotors, and master cylinder. Nothing helped.

Finally, I corrected the problem myself by carrying a 10mm open-end wrench with me. Each time the brake started to drag, I loosened the brake line to the front brakes at the master cylinder and bled out a few milliliters of fluid. The excessive drag on the brakes would cease immediately. After doing this 4 or so times, the problem disappeared and has never returned.

This year, the car has been on two lengthy trips, including one through the Appalachian Mountains, with no reoccurrence of the brake problem. It does not pull straight under braking at times, but there is no abnormal brake drag. It looks like the problem is solved.

Does anyone have an idea of what caused this problem?

[vandor]

Did the mechanics adjust the length of the pushrod between the MC and the booster?

[Coolbreeze]

Yes. More than once.
You gotta admit this is a weird problem. The solution is just about as weird. I'm just glad the problem went away -- but I keep the wrench in the car just in case!

[RRoller123]

just a small possibility..... I noticed when I put on new front disks that one didn't sit quite flush and flat at first because of some crap on the hub. The little, almost imperceptible wobble caused the brakes to slightly drag, and the hubs were always hot after I would check them after a drive. I kept thinking it was the wheel bearings. I suppose if this were to cause enough heat to boil the brake fluid, these symptoms could occur? Just wondering. Could a permanent slight disk wobble cause this?

[Coolbreeze]

I doubt it because the brake would not have freed up immediately after I released a small amount of brake fluid.

The brake on right front was really hot. I carry a gallon jug of water and towel on trips to keep myself clean and fresh. I splashed the entire jug on the steel wheel. It was still producing steam when I put the last of the water on it. The left front was hot too, but not nearly as much.

Thanks for the suggestion.

[RRoller123]

If you doused the disk when it was hot, to the point that steam came off, then it unfortunately is likely now badly warped and will have to be replaced.

[Coolbreeze]

I knew I was taking that chance, but I wanted to cool the wheel before damaging the tire, caliber, or frying the axle grease.
As it turned out, the rapid cooling caused no harm.

[spider2081]

A few thoughts

When you say your "replaced the lines" do you mean the hoses to the calipers from the solid lines? Or have you replaced the solid lines and the hoses. It sounds to me like a defective hose. I have seen hoses that will allow the brake fluid to pass under pressure when the brake peddle is pushed but too restricted to allow the fluid to flow back out of the caliper when the pressure is removed.
I would first remove the solid line fitting from the hose in the wheel well. The fluid should flow freely just from gravity. If it doesn't remove the reservoir cap. If then it flows then the cap vent is clogged. If it doesn't flow free with cap off then there is a restriction in the solid line. If the solid line is clear then I think the new hose is suspect.
Are the caliper slides lubricated with a brake caliper grease? Dose the caliper appear free if you try to move it in its mount.

[Coolbreeze]

I had heard of old brake hoses acting like check valves; so, I had the hoses replaced also. I left the metal lines in place because they were unlikely to cause the dragging brake problem.

The rebuilt calipers came from IAP; so, they should be good. IAP even replaced the first caliper I got from them after the problem started. The replaced caliper did no better.

The two things that should tell us something is that the problem arose after I replaced the brake booster and it went away after I removed ~1cc brake fluid by loosening the line to the front brakes at the master cylinder each time the brakes started acting up. With great clues like these, you would think the mystery could be solved.

I even wrote to "Fix It Again, Tony" in Hemmings Sports and Exotic Car magazine for advice. So far, I have heard nothing from him.

BTW, I'll share a near disaster experience caused by the brake problem under the "Fiat Travels" topic when I get around to writing about it.

Meanwhile, Happy New Years.

[spider2081]

It really sounds like the fluid is restricted and can not freely flow back to the master cylinder when the brake peddle is released. That is why I asked if the brake fluid flowed freely out of the metal lines with the hose fitting removed. It should flow with a good stream just from gravity.
You might disconnect the metal lines at both ends and blow them out with air from the caliper toward the master cylinder.
Are you positive the metal lines are connected to the master cylinder in the proper places. I don't know what would happen if a front line was installed where the rear line belongs.
These are just some thoughts

[Coolbreeze]

Good suggestion. If the problem still existed, I would blow out or snake the metal lines. Right now, "If it ain't broke, don't fix it."

I'm pretty sure the lines are connected correctly to the master cylinder because the brakes work as they should.

Interesting mystery, eh?