Brakes locking up

[18Fiatsandcounting]

I've read a number of posts from people who experienced their brakes locking up after driving a short distance, and the proposed issue is often a misadjusted acorn nut from the booster to the master cylinder. While I have never had this problem, I have to admit to not fully understanding how this happens.

Here's the typical scenario: Work is done on the brake system, it is put back together and seems fine, but after a relatively short drive (10 miles?), the brakes start dragging. In my way of thinking, either something is increasing the hyraulic pressure in the brake lines over this time, or something is heating up (expanding) and causing the brake pads to push against the rotors.

So if the acorn nut was misadjusted and you bled the brakes and they seemed fine, what causes the lockup later on? I'm not saying you shouldn't adjust the acorn nut, just that I think there must be something else going on here.

Possibilities:
1) The calipers are not freely sliding in their holders, so when the brakes are applied, the pads don't fully release and start to drag. The pads heat up and the problem gets worse.
2) The caliper pistons are not retracting like they should, so again, either or both pads drag slightly, the pads heat up, and the problem gets worse.
3) My understanding is that the booster normally has vacuum on both sides of its diaphragm to equalize the pressure, but when you push the brake pedal plunger in slightly, the booster reduces the vacuum in the rear and so the booster "helps" further movement of the brake pedal plunger. Perhaps this system isn't working as it should?

Any thoughts? Just trying to fully understand this issue.

-Bryan

[SteinOnkel]

Another possibility may be that the pistons aren't pushed back all the way when the pads are swapped. Then they are installed with a bit of force and there you go, dragging brakes.

[18Fiatsandcounting]

Yes, that would do it, and I thought of another (fairly common) possible cause: The brake hoses are semi-collapsed so that you can push fluid through them, but then they maintain some pressure on the calipers after releasing the brake pedal. I had this happen on my '69 Chevy truck that had a collapsed rear hose, and the brakes would gradually lock up in the rear, and release only after 5 or 10 minutes of not using the brakes. A new brake hose solved the problem.

One way to check all this is to stomp on the brake pedal a few times while the car is just sitting, then release the brakes and see if you can fairly easily roll the car by yourself. On a level surface, you don't have to be especially buff to be able to roll a spider by hand. Assuming the brakes aren't dragging of course.

-Bryan

[Nut124]

I have had this happen on a few occasion over the years. Typically, a few minutes after start of driving, usually on a highway rather than on a low speed street, I notice that the brakes are dragging and the car wants to slow down unless I apply more gas. I usually pull over and pump the pedal hard a few times and drive on. I have never actually diagnosed or fixed this.

[seawood]

If the acorn nut protrudes too far, the brakes will work when pedal is pushed but the brake fluid is not released inside the MC when the pedal is released. In effect you are building pressure in the brake lines and the pistons will remain locked.
While fiddling with my MC the other day after replacing, I backed a bleed screw off and let a little bit of fluid out. Released the brakes.

[RRoller123]

[18Fiatsandcounting]

I'm still not sure I fully understand the acorn nut/booster/MC adjustment being the cause of brakes locking up. If this were the case, "riding your brake pedal" would also prevent the master cylinder from relieving pressure, and as far as I know, riding the brake pedal does not cause the brakes to slowly lock up. (it does cause other issues of course)

Suppose the acorn nut is way out of adjustment so that the pistons inside the MC are partially compressed even when the brake pedal is fully released. You bleed the brakes, and they now work. Now you go for a drive, and when you press the brake, the MC pistons pressurize the pistons in the calipers, and the brakes work. When you release the brake pedal, the MC pistons retract to their partially compressed position, and the caliper pistons relax and the brakes are off. What's causing the pressure to be maintained in the system when you press and release the brakes?

I can see an issue if the acorn nut is way, way out of adjustment so that the pistons can't retract enough to allow new brake fluid to bleed in from the reservoir (if needed), but that would be an extreme case. And it doesn't explain the slow lockup.

And remember, we're talking about acorn nut adjustments that are specified in the few thousands of an inch tolerance, when the MC pistons move much more than that in normal operation. A quarter to half inch perhaps?

-Bryan

[seawood]

What I did was looked for a straight edge, used a set of Vermeer calipers, and with the MC pulled aside placed the straight edge vertically against the booster next to the booster rod. I have a plastic cup that holds the booster rod in place and pushed the straight edge up tight against the plastic. It does recess for the most part into the booster, and then I used my mark1 eyeball, with corrective lenses, and could see where the tip of the acorn nut was in relation to the straight edge.
Did my best to guestimate the 1 millimeter distance and locked it up and put it back together. Did this evolution twice more with half turns on the acorn to get the brakes to work and at the same time release when the brake pedal is released.
I don't know if booster rod lengths vary, or if MC recesses differ with different brands, but with a bit of fiddling about, one can set the brakes up after bleeding, including rears by suspending using the axle and not the frame, without too much fuss.

I have a thought, when adjusting the rod length, how about considering using a bit of lipstick, when better half is out shopping or something, and putting a swipe inside the MC recess. Assemble, take apart and see if there is any on the acorn. Yes, back off the acorn until none is showing... Someone want to try?? Any thoughts from the long time Fiat techs? ( Beek)

I understand if you assemble with the acorn depressing the MC( out too far), you in effect have a partial brake application and the fluid is only going one way. Brakes drag. Press the pedal and you introduce more fluid into the lines and the car stops/slows down. However when one releases the pedal, the MC still has the acorn pushing into the MC and the fluid does not return/release being a partial brake application. Lines are pressurized so back the bleed screw off and release the pressure.
Clear as mud I'm sure