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MG MGB Technical - Weird Brake Lockup

I had to take a cell phone call and pulled off onto a gravel driveway. I talked on the phone for 6 to 10 minutes. During that time I left the engine idle and had my foot on the brakes. When I tried to drive off, I couldn't move. I could feel the driveline take up slack and try to move the car, and I thought that I may have pulled up to a brick that was in front of a wheel. Tried reverse, and felt the driveline slack go the other direction but the car wouldn't move. I then selected first again and gave much more throttle to try to "break it loose." One of the rear wheels spun the gravel a little, but it didn't move. I did notice that the brake pedal was very "hard." I could press it, and it would be rock hard just after the mechanical slack in the pedal was taken up. I looked at the brake fluid level and it was ok. I scratched my head (and other parts) but was stumped. After about 5 minutes I started it again, and the brakes were fine. I never applied the parking brake, and all hoses were replaced a few years ago, as were front discs and a couple years ago the rear shoes and cylinders. Brakes have been fine till this weird incident. I'm thinking it may have to do with the long application of the brakes while I was sitting, coupled with the heat of the engine idling. BTW the engine never overheated or even reached the "H". Any thoughts?
David Burke

Perhaps the brake pedal is binding on the shaft in the pedal box?

The only other things that would maintain pressure are:
- sticking valve in master cylinder
- 'chevrons' in the brake hoses, but it doesn't sound like that at all.

Actually, my guess would be master cylinder, but the pedal box costs nothing to check!
D Ainsworth

Do you have a servo/booster?
Dave O'Neill 2

Also could be a bit of grit blocking the return hole between the master cylinder and the reservoir, causing pressure to build in the system due to heat. I had this problem on a Toyota several years ago. Once cooled down a bit, the problem would disappear.
Erick Vesterback

Thanks for comments. The car has no servo, and the pedal does seem to be free. Brakes did work fine after the incident on the short ride home. I am getting ready to drive it today and will see if it does it again. I also plan to change the fluid because this incident reminded me that the fluid hasn't been changed in several years. This might help if the problem is a bit of grit, but even if not, it needs to be done. If problem continues, you'll hear more from me!
David Burke

David,

You didn't say what kind of MGB you own.

If it is new enough to have a dual line brake system, and if it were my car, I would drive it cautiously for a while and see if any weird symptoms arose.

Charley
C R Huff

Hi
I had the identical problem after pulling off an interstate onto some loose gravel.
Front wheel would not budge.
I cracked the bleeder and all was fine.
I then found a huge roadside pull over and ran up and down a few time times spiking the brakes and everything worked as it should.
All I could think of is a rock had got caught up and would not allow the brake piston to retract.
I checked out the brakes when I got home and was unable to find anything wrong. Now 2 years later and no problems.

FWIW
Bruce

Bruce Mills

I did a track-day recently and the instructor said not to put the handbrake on when coming in after a run. Obvious really, the discs are hot and having the brake engaged helps transfer the heat to the caliper and hence the fluid. Result is boiled fluid, air in system, no brakes next time you need them...

I wonder if this problem was similar in principle? Keeping your foot on the brake would put all of the pads/shoes into contact and perhaps caused something funny to happen because the fluid heated up and increased the pressure? Or even the pad somehow stuck to the disc due to the heat?

Neil
Neil

Neil might be onto something.
I did a stop on an incline from highway+ speed and sat beside the road for a bit with the brakes on while I figured out where I was. The problem I have always had with the rock theory is that I never did see or hear a rock fall and my face was right there beside the wheel!!


Bruce Mills

Dave

One question. Is the brake switch located on your brake pedal box? If so, and it is just slighly in to far, it will not allow the brake lever to move far enough to open the brake fluid hole.

Took me a couple of days to figure out why my brakes were not releasing after I put in a new brake master cylinder. A call to John Twist gave me the recommendation to unscrew the brake light switch just a little.
Bruce-C

This thread was discussed between 10/10/2008 and 15/10/2008

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