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MG MGA - Brakes locked on, please help

Hi folks. I have an MGA 1500 that has beens converted to MGA 1600 Lockheed disc brakes up front. This conversion has worked well since 1980. I recently did a complete hydraulic system overhaul on my car. All new copper nickel lines, new brake hoses, rebuilt calipers, and rebuilt wheel cylinders. A freshly rebuilt master cylinder is also installed and is causing me a problem. In 1980, I purchased 2 brake master cylinders, one a proper 7/8 inch bore MGA unit, and the other a 3/4 inch bore bugeye sprite unit. When I installed the MGA unit back in 1980, the brakes locked up, and would not release. Frustrated, I then installed the bugeye master which worked fine. I rebuilt the MGA master cylinder unit last week, making sure to enlarge the front cover gasket piston holes, to eliminate the covered port problem, which I learned a few years ago can cause the brakes to lock. Even with the front cover gasket suitable relieved, the new brakes lock on, and wont release. Loosening a bleeder screw allows the brakes to release.
John Twist suggested that I unscrew the screws holding the front cover on the master cylinder. Doing this also allows the brakes to unlock, but then when I touch the brake pedal, the brakes lock up again. Loosening the front cover a bit more frees up the brakes again until I touch the pedal, then the brakes lock up again. Clearly there is a problem with the master cylinder. It pushes fluid to the brakes, but apparently is not allowing fluid return. Has anyone have a similar experience, or would anyone care to comment on probable cause of my brake lock up? Does my master cylinder have some sort of casting or machining flaw that is causing this problem? Thanks! Glenn
Glenn

Glenn,
It has been a long time since I rebuilt my master, but I believe there are two holes from the reservoir for each piston. The one toward the front fills the space around the piston with fluid. The hole toward the rear relieves pressure when the brakes are released. This is the hole that doesn't get uncovered by the piston if the rod is adjusted too tight, but in your case it sounds like this rear hole is either clogged or not machined properly.

Jeff
Jeff Schultz

Glenn

Jeff's explanation rings a bell with me. I recall about 9 years ago when I rebuilt my car that I turned a few thou off the front face of the brake piston for this very reason. I believe it was a tip from Bob West or James, his mechanic. It might have been as little as 6 thou. I am phoning him in the morning for some parts so will check to confirm.

Steve
Steve Gyles

Glenn,
Have you tried adjusting the brake push rod. When I replaced the master cylinder in our several years ago we had a problem of the brakes hanging up. The brake drums would become to hot to touch after a short drive. After adjusting the rod everything was fine. Just a thought.
Tony
Tony Shoviak

Glen,
What do you call 'locking on'. Can you push the car? At the end of the brake cylinder there is a one way valve that is intended to keep a small amount of pressure in the system when the brake is not being applied. The pads always touch the discs with varying amounts of small pressure - unlike modern discs where the pads draw back more. One check is to drive the car and get the discs hot, jack up the wheel and see whether with time the wheel begins to come free.
J H Cole

Glenn

I have just spoken to Bob West. He said this is quite common especially when using silicone fluid due to an initial slight expansion of the rubber. This causes the pressure relief hole to remain covered. He solves it by taking 20-30 thou off the front face of the piston.

I remember following his advice and have never had any problems. I also use silicone fluid and swear by it as well you know.

You are not getting much driving up there this year!

Steve
Steve Gyles

I had the same problem with both my 1600 roadster and 1600 coupe when I used to use silicone brake fluid. I took at least 60 thou off the face of the master cylinder piston (so that it comes to rest that much further to the front of the car) about ten to fifteen years ago and later switched back to ordinary brake fluid with absolutely no ill effects..
Barry Bahnisch

Hi folks, thanks for the ideas and comments. As a temporary fix I will rebuild and reinstall the bugeye master cylinder. When I have more time, I will dismantle the MGA master cylinder, and figure out what is happening. Cheers, GLenn
Glenn

This thread was discussed between 21/07/2006 and 24/07/2006

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