Weird Brake Problem
 

Posting for a friend - has an '83 SC. Replaced Brake MC, Lines, Rotors and installed rebuilt calipers. Bleeds brakes - appears to get good firm pedal. With no brakes applies, rotors spin fairly easily. With leting the car sit overnight, no changes, no brake pedal movement, still on jack stands, comes back next morning and the calipers have locked up - rotors will no longer turn. What's going on here?

The return flow is prevented by a blockage, swollen internal hose, breather tube (at MC?), or otherwise. Put a tube on each caliper bleed nipple in turn and open it up just to bleed off pressure (no brake pedal), checking again to be sure the rotor is spinning free. Test by pumping, releasing, checking rotation. Ensure all vent tubing is clear. Let sit overnight and see if it returns, if so, rebleed.

Sorry not to be of more help - others will follow.

EDIT: Tap the calipers when you vent them to remove air.

my guess would also be the rubber brake lines have swelled up inside and won't let the fluid return.

Lines are new. Could there be blockage in the hard lines?
What happens to cause the calipers to, over period of time, lock up?

Just to clarify, all 4 calipers are locked up? What method did you use to bleed the brakes (Pressure bleeder, vacuum, pumping the pedal)? Was the MC new? Was it a factory part?

"..the calipers have locked up...."

"how locked", how much muscle to get them to turn....?

Not what you're asking, but....rotors should NEVER turn easily after use/application.

Friend said the calipers locked up tight enough that he needed a pry bar to move them.
If memory serves, he bought this car after it had set for a while and had problems with the brakes dragging which is why he decided to do a redo of the hydraulics and pads. Would seem the only thing left would be pedal cluster (which he rebuilt) or something with hard lines. Does SC have a proportioning valve?

Still seems weird the pads would move/clamp all by themselves. After bleeding, the brakes appear to work and release an appropriate amount (can move the rotors pretty easily). Then they clamp down while sitting over night.

my car did a similar thing (not trying to hi jack, not looking for a fix). i back it in the garage, then a week later went to push it and it would not move. finally got it to move but there was a loud, audible, pop, then it moved. my calipers are in need of a rebuild, they leak somewhat.
i found one piston on each caliper stuck when i got my car, now i use the brakes hard enough to keep them from sticking.

perhaps just bad calipers, but all 4?? and yes a saw they were rebuilt. defective MC?

i was doing brakes on something not long ago, think it was the jeep, i noticed the pistons pulled back in when the pedal was released.

see if it does it again, may have been a freak thing.

So, after using the pry bar initially was he able to more easily turn the rotors, say by hand..??

If so that would imply some sort of "sticktion" which might turn out not to be a problem at all.

That "loud, audible, pop.." described in post # 8 is pretty common IMMHO. I think it is the result of parking the car with the pads/rotors being fairly warm and then as they cool overnight they stick together.

I don't know as I have ever tried manually moving the rotors, by hand, under the conditions you describe. But as I have said the "condition" may be quite common and have no long term ill effect at all.

Hmmm....New, SMOOTH, rotor surfaces, new, SMOOTH, pad surfaces.....

What you describe seems entirely possible, plausiable...

My first thought is hoses swelling,but you have replaced all 4 . Next thought is calipers but they are all rebuilt. That probably leaves the master cylinder. Either the pedal cluster is holding the brakes applied slightly or the pushrod length between the booster and MC is set too long resulting in a relief port in the master which may be covered .

an idea - on a wheel that is locked without the brakes applied, trying bleeding and see if there is residual pressure, then try moving the pistons, if bleeding releases pressure then the problem is upstream of the caliper

If he messed with the pedal cluster or the MC, or both, he might have ended up with the MC actuating rod being too long. Or the pedal not completely coming back far enough.

The spec is that there is a little bit of clearance between the end of the actuating rod and the cylinder inside the MC. There is a spec, but all you need is some clearance. This insures that the pistons inside the MC can return all the way to the rear (of the car). If they are a bit forward of fully retracted, they block the hole to the fluid reservoir.

If that hole (holes, actually) is blocked, the pressure in the brake lines can't be released. Yes, if the pistons move back, that removes some of the pressure. Not, I think, back to zero. And especially if there were still some air in the lines/calipers.

Since the caliper pistons do their initial returning due to the flexed rubber inner seal returning back to its unflexed position, I don't suppose it takes much pressure to keep the pistons a bit out, with the pad pressing some.

Plus, with the MC holes blocked you can't really bleed the system.

Just something to check.

If all 4 wheels are locked then the failure is in a common portion of the system. The odds of all four corners failing in the same manner is astronomical.
In other words: The failure is between the pedal and the master cylinder.

Walt's post pretty much covers it. Most likely it is master cylinder retraction. I'm wondering if the power brake booster is involved here. I can't remember how the rod goes from the MC to the booster. If it isn't seated correctly the MC won't retract completely and the pressure can't release. Also if the pedal linkage isn't releasing completely (so there is some freeplay) then the booster will be partially activated.

Thinking about the "It was free last night and now it is locked" bit. That would require a movement of fluid or an increase in fluid volume coupled with a MC that isn't releasing pressure.
- The only realistic theory that I can come up with why the "quantity of lockup" would increase overnight is that the reduction in temperature increases the level of trapped vacuum in the booster and increases the booster's level of activation.

Other possibilities:
- Moisture or other contamination of fluid is expanding (evaporating/freezing) inside the lines causing an increase in pressure.
(I just don't think this can happen unless a quantity of water is freezing causing the expansion while the MC isn't releasing.)(Getting cold in Florida?)
- The slight contraction of the metal lines from overnight cooling is causing the clamping pressure.
(I can't believe that this is even possible...)


End result: Check the MC. Loosen the bolts mounting the MC a few turns until you can feel it and its pushrod is loose and see if that frees the wheels. If so get the linkage right. If it is still locked then the MC is bad.

i moved my car from one garage to another, about 50 yrds.

i have been pushing the car around, just to move it (it runs), they have not locked up again.