car spun when brakes locked

[fgross2006]

So I was cruising my 84 LX convertible on my way to a doctor appointment when I had a harrowing experience.

For those on Long Island, I was cruising Jerusalem Ave in Levittown northbound. For non Islanders, its a four lane road, two in each direction separated by a median.

There were no cars on either side of the road and I was cruising a comfortable 45-50 carefree. Then out of nowhere a UPS truck darts out of a side street onto Jerusalem Ave in front of me. I had about 50 feet between us, but I instinctively stomped the brakes. I immediately fishtailed and spun 180 degrees, went over the median and landed on the opposite side of the road, now southbound facing the correct direction.

Lucky that there was no southbound traffic or I would have side slammed an oncoming vehicle. Also very lucky the median was grass and not cement. There was a curb around the grass but it was low and my car slid right over it, the grass and dirt created skidding wet road effect which caused my spin to carry me to the southbound side.

No damage at all, thankfully. Just grass imbedded in my passenger front tire rim bead.

So can anyone tell me why my car spun when I locked the brakes? I know these old Stangs don't have ABS, but I'm pretty sure they were not built to spin out of control in an emergency stop.

I did have my proportion valve apart earlier this summer and did have issues with the low brake fluid sensor leaking brake fluid so its been capped with a plug. I do plan to open it again since the guy I got the o ring kit from sold me a new brass piston and the brass sleeve that mine didn't have in it. But if I'm not mistaken, that part of the combination valve is for the front brakes. My mechanic told me the brake pressure increased to the rears after the proportion valve rebuild. I assumed the spinout was caused by the rears locking, unless I'm wrong.

So I have new front brake pads, new rear shoes, new master cylinder and a proportion valve that was disassembled and needs another reassembly.

[KevinK]

Locking the rear brakes should not cause much except a slight squirly movement. Car should maintain a straight direction. May be shift to 1 side 12". I test m6 Foxbody brakes to make sure the rears come close to locking up. Many people curse the Foxbody brakes only to find the rears are not adjusted properly.

I think you have something going on with the front. Alignment, tires, calipers, tie rod ends, wheel bearings?

[richpet]

It is because the rear brakes locked and your fronts did not.
If the rears lock and fronts do not and weight balance is 100% equal left to right you will stay more or less straight.
I taught police pursuit driving for more than 20 years. A sliding tire in essence tries to go faster than a rolling tire.
Super light rear end compared to the front. Weight is what makes tires work. No weight, no ability (in comparison) to work.
The panic hit of the brakes is a very abrupt thing which also contributed. Having any turn at all to the front wheels exacerbates the issue. Or, anything that can cause the fronts not to be perfectly straight - alignment, worn parts, etc.
Glad you didn't get hurt!
I had the same thing happen in my old '72 Capri, but it wasn't the brakes. Driving on snow to work. Downshifted to 2nd while turning ever so slightly. Rear tires spun (slower than the car due to the snow. Essentially the same as you locking your rears - they were not rolling but sliding) and I did a 180° through the intersection.
I can't even begin to guess the number of times I have seen this unintentionally demonstrated on the track...

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[Broncojunkie]

I know your rear brake shoes can get really "grabby" when the cylinders leak. It doesn't take much. Some weeping is normal, but even that can build up enough to be noticeable if not driven daily.

[gr79]

Great to hear your experience did not result in damage or injury, just a lifetime memory.

Road conditions, mechanical condition, and state of mind have a lot to do with what happens.
Spun mine 2x back in the 80's at 60 or so.
Long off ramp, s curves with two bridges. Night time, November Thanksgiving eve.
May have hit black ice. There was snow on the ground but roads were clear.
Have the ugly pic of the car on drive Thanksgiving Day. Was drivable.
Hit brakes hard before 1st curve. Stock brakes. Rears locked up stalled the motor.
Left front hit concrete barrier wall. Did drive it home after prying fender off wheel.
The s wheel spun so fast back and forth i let it go just like race drivers say to do.
Later found the car could take the same curves when dry, at that speed, no problem.
Tire marks and paint scrapes on a wall or curb anywhere on freeways are reminders.

Car is lower at ds fender lip by 1/2" even after collision repair by dealer.
My car rotates to the right on panic stops or to fast stops on wet roads.
Brakes are like that. Sticky caliper, uneven self-adjustment of drum brakes.

[fgross2006]

Locking the rear brakes should not cause much except a slight squirly movement. Car should maintain a straight direction. May be shift to 1 side 12". I test m6 Foxbody brakes to make sure the rears come close to locking up. Many people curse the Foxbody brakes only to find the rears are not adjusted properly.

I think you have something going on with the front. Alignment, tires, calipers, tie rod ends, wheel bearings?

All that stuff is new or fairly new. Just put new tires on this summer, had it aligned. The entire front end was re done with all Moog parts and the alignment shop said all was good with the front end.

[fgross2006]

I know your rear brake shoes can get really "grabby" when the cylinders leak. It doesn't take much. Some weeping is normal, but even that can build up enough to be noticeable if not driven daily.

brand new wheel cylinders put in this summer.

[85GTGuy]

Being as you have quite a bit of new stuff (front end parts, rear brakes) on your car, I think it was uneven braking, front to rear... like richpet said. Imo, that points to the proportioning valve... especially since it's been worked on.

[KevinK]

Please check your front wheel bearings. I have seen the nuts strip enough to loosen.

[fgross2006]

Here's an update on my brake situation. Today I pulled the proportion valve again and replaced the brass piston and the brass sleeve for the front part of the valve. It literally had no positive effect on the loss of pressure through the low pressure sensor. After re installing the proportion valve I had my son help me to bleed the brakes out. We started by bleeding each line that goes to the proportion valve and it released plenty of air.

I bled the fronts, no air came out. Bled the rears, again no air come out but I noticed that my rear passenger side has almost no brake pressure. That has a new brake cylinder too. When I open the bleeder it drools and my son said the brake pedal did a slow crawl to the floor. Instead of a the quick drop when the bleeders are opened.

So; my proportion valve is still not functioning as it should. After bleeding out the system I removed the set screw that holds the internal piston then put my low brake pressure sensor back in. As soon as my son hit the brakes it pissed brake fluid in a straight stream through the top of the sensor. Although there is a tiny steel tube between the prongs for the pigtail, I'm sure its not supposed to allow fluid to pass through. I was really hoping the new parts would solve my problem but no good. Very disappointing.

I've gone as far as I can go with the original proportion valve. It just cant be made to function. And since I fishtailed more than once, I can assume that may be part or all of the problem. I am going to have my mechanic run one new brake line from front to rear for the passenger side. There has to be sludge or some obstruction. As for the proportion valve, I am going to take my chances with one of the after market ones I see that are supposedly for 79-86 Capri. How much different can it be to a 79-86 Mustang brake system?

[fastsvo]

I know your rear brake shoes can get really "grabby" when the cylinders leak. It doesn't take much. Some weeping is normal, but even that can build up enough to be noticeable if not driven daily.

How much weepage is considered ok? My car sits a lot and did notice some weepage after I touched the wheel cylinder seals.


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[Bentley]

When you bleed brakes you always do the farthest line first, which is usually the right rear. The the other rear, right front then the left front last..
I have seen gravity bleeding done with ALL of the bleeders open, then closing each as the fluid ran clear with no bubbles....

[richpet]

When you bleed brakes you always do the farthest line first, which is usually the right rear. The the other rear, right front then the left front last..
I have seen gravity bleeding done with ALL of the bleeders open, then closing each as the fluid ran clear with no bubbles....

Shops do similar with a low pressure tank on the master to slowly force fluid.

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[massacre]

Shops do similar with a low pressure tank on the master to slowly force fluid.

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I use a pressure bleeder, what a godsend when working alone.

[85GTGuy]

.... Some weeping is normal,....

I'm gonna have to disagree with that. If everything is prim and proper, and being it's a sealed system, I wouldn't expect any amount of leakage, no matter how small, imo.

My car sits a lot and did notice some weepage after I touched the wheel cylinder seals.

Again, imo, a wheel cylinder seal should not leak. If just touching it, compromises it, then that seal is not capable of doing what it's supposed to do.

[richpet]

Agree. No amount of brake fluid seepage is a good thing. If fluid can get out, moisture can get in. That is the enemy as moisture lowers the boiling point of the brake fluid.
Seepage means something needs fixed.

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[qikgts]

Any success fgross2006?