Brakes locking on.

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Brakes locking on.

Postby bugeye1600 » Sat Dec 28, 2013 11:51 pm

Ok folks, here's a technical question for you.
My Bug runs a complete custom braking system, consisting of:
-Gillspeed front disc brake conversion, featuring Ford Cortina calipers with 9" discs;
-Ford Anglia rear brakes, featuring 7/8" wheel cylinders;
-Ford Laser master cylinder, 7/8" bore.
Otherwise the set up is as per standard, i.e. no booster, proportioning valve, etc.

Here is the problem:
After about 18 months of the project being idle, I needed to relocate the Bug and, as it is nearly finished, and the journey to it's new home was only about 5 miles, I decided to drive it.
I got to within sight of my destination and felt that my brakes started to lock on. I pulled over to the side of the road, jacked up the front and checked the brakes, which were very hot. After they cooled down a bit, I was able to get the Bug going to it's destination. I stopped outside the garage to open the door, turned the engine off and had real trouble pushing the Bug inside by hand.
Next day, I was able to roll the Bug around the garage freely.
I got some advice from several sources. The consensus was it was calipers. I dismantled them. The pistons were corroded, so I sent them off to be rebuilt. Re-fitted them and, before attempting to bleed the brakes, I pressed the brake pedal. It was rock hard. Great! Master Cylinder, I assumed. It turned out that I had a spare, which I fitted. Then, I bled the brakes and, guess what? With the Bug still on the stands, the front discs are locked on!
Interesting point: When they locked the first time, I checked both wheels on the side of the road. The near side one was not as tight as the offside. After I bled them, the same situation occurred!
I checked the rejected master cylinder and the piston moved freely in the bore. So, assuming the calipers were re-kitted correctly, the change of master cylinder did not solve the problem, all the solid brake lines, rear flexible brake hose, rear wheel cylinders, shoes, etc are new, is it possible that the situation is caused by the front flexible brake hoses? Any other ideas?
I should also add that I have driven the Bug several times over the years, in varying stages of completion and the brakes worked well (for non-assisted brakes).
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Re: Brakes locking on.

Postby Charlie Tolman » Sun Dec 29, 2013 1:42 am

I had a situation with a non-Sprite car, where a pair of rear brake shoes (LR) would be totally destroyed in 200 miles, since they were dragging. The brake drum would not turn easily.
This problem was eventually solved by replacing the flexible hose to the offending LR brakes.

The flexible rubber hose at the LR apparently had deteriorated internally, creating a blockage. When the brake pedal was pushed, there was enough line pressure to force brake fluid through the restriction in the hose, and the brakes shoes would move. However, when the brake pedal was released, return springs for the brake shoes did not have enough force to cause the pistons of the wheel cylinder to move back to their original position in the cylinder, since the hose restriction was blocking the reverse flow of the (low pressure) brake fluid. This hose restriction was acting as a one-way flow valve.

Your car might have a similar problem with the disk brakes ?

Charlie Tolman
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Re: Brakes locking on.

Postby bugeye1600 » Sun Dec 29, 2013 4:08 am

Hi Charlie,
Thanks for your reply; it makes a lot of sense. Through a process of elimination, I concluded that the flexible hoses could be the culprit but couldn't imagine how. I did forget to mention that depressing the brake pedal to bleed the brakes took a fair bit of force, which supports your theory.
It's back to the brake shop for me!
I will report on progress.
Cheers
Steve
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Re: Brakes locking on.

Postby Charlie Tolman » Mon Dec 30, 2013 1:45 am

Steve,

On my car, there was not any external evidence on the LR flexible rubber brake hose that
there was an internal problem. $12 and some time, fixed the issue.
Interestingly, the RR flexible brake hose never had the blockage problem, even after a few additional years of driving.

Charlie
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Re: Brakes locking on.

Postby bugeye1600 » Sun Jan 12, 2014 12:59 am

Well, I finally managed to make time to get the front brake hoses off, get down to the brake place to have new ones made, go back to pick them up, then back home to re-fit them and bleed the brakes.
Then, it was a case of finding the time to move 3 or 4 other cars around the shed to be able to test drive the Bug, which finally happened this morning.
The result?
Problem solved. The brakes worked excellently, pulling the Bug up very well from quite fast speeds. In fact, I don't think the brakes have ever been this good. All for the cost of $A75.
So, thanks SpriteSpot and, especially Charlie, for helping to solve the problem.
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Re: Brakes locking on.

Postby Charlie Tolman » Sun Jan 12, 2014 1:33 am

I am pleased to hear that new front flexible lines solved your brake problems.
You're welcome.

Charlie T.
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Re: Brakes locking on.

Postby mmacquee » Wed Jan 15, 2014 3:59 pm

a lot of people neglect to swap out flex hoses periodically. I saw this issue a lot when I workd as a mechanic (years & years ago). When you cut through such a hose, you will see it is almost completely occluded. It's another argument for using braided steel comp-type hoses on everything, they are not subject to such degradation.
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