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MG MGA - Rear wheel locking up when braking.

Guys,

On the weekend I had to replace the rear flexible brake line as it caught itself over the battery box and began to cut through it.
We replaced the line and bleed the system starting at the back and working forward. The pedal pressure was firm. The rear right wheel was very tight on the brake when we finished. I took it for a test drive and when I braked quickly the rear wheel/s would lock up first. Went back to the shed, the rear right wheel would now turn freely. But still when I brake quickly the back-end locks up first, enough to scare me and let me know something is not right.
Nothing else was changed; no pads or drums were removed.
All ideas appreciated.


I might suggest anyone who has a 12v battery in the cradle next to this flexible line may want to check its not caught on it.
DJ Lake

Is the brake adusted correctly? Also, while you have that wheel jacked up, get someone; a wife will do, to operate the brakes firmly whilst you check the wheel by turning it to see if it frees as soon as the brake pedal is released. Report what you find, Lindsay.
Lindsay Sampford

DJ

I think you you will find this is not an uncommon experience in the A. I asked exactly the same question on 4 Oct 2006: http://www2.mgcars.org.uk/cgi-bin/gen5?runprog=mgbbs&access=&mode=archiveth&subject=6&subjectar=6&thread=2006100419521827700

The answer from one authorative MGA owner/ garage owner was the following:

"This is a common problem with A's, mine used to do it until I improved the front brakes, try filing a leading edge onto the rear shoes, this should help reduce grab. Try Bob, I think he sells harder shoes. You could fit an in-line compensator."

Steve
Steve Gyles

Steve, Before replacing the line the brake balance was fantastic, no lock-ups, straight braking with a soloid stop and pedal. Im not sure I should adjust the back brakes outward. Will this help or simply reduce the brakes ability completely.
DJ Lake

So DJ are you saying that the rear wheel turned freely before you changed the brake line but was tight after you changed it?
Lindsay.
Lindsay Sampford

I had a simmilar problem on my 1600 A. As it turned out, the brake line was colapsed internnaly which was acting like a check valve. Your new line may have a problem with it that is causing this problem. The brake fluid is not flowing back to the master cylinder the way that is should thus causing the lock up. I also had a rear brake that would lock up. This was caused by a broken adjustment cradel. this is the small U shaped piece that holds the adjustment cam. The U bracket broke and cocked sideways, this caused the brake shoe to drag and when it got hot, would act as if I was stepping on the brakes.
JP Jim

I don't think I did anything specific to cure my locking up. It just seemed to be a 'winter' thing when the tyres were cold - less grip. Also, the MGA is I guess relatively light at the back compared to the front, unless you keep the tank topped up!. That said, I did suffer a MC cylinder leak at about that time and, on recollection, the locking-up did not recur after I had carried out a full overhaul.

Locking up is something we see with the F1 cars - no grip when the tyres get cold.

Have you had a recent big drop in temperature down under?

Steve
Steve Gyles

Any fluid on the shoes can cause rear wheel lock up. Diff gear oil or brake fluid.
R J Brown

Another scenario is that the hose was blocked and the rear brakes were not working, or working slowly. That would hide the common error wherein the shoes are put on backwards, making them have way too much self servo action. Can't recall if the MGA shoes CAN go on backwards, but this is very common on MGB and Spridget. Both shoes are meant to be trailing when going forward, and leading in reverse. If you notice that stopping is very poor on a quick backup, that's why.

Also it is possible that you have, by disturbing the system, caused the front brakes to work relatively less well, or slower in response - deteriorated hoses, or rubber bits lodged in pipes. Recently had an MGB that did just this, due to random trash in the pipes from bad hoses and MC seals. The thing had had no rear brakes for an unknown time, due to crushed steel pipes and a bad hose. Few drivers ever notice this. After all was reassembled with all new rubber, the front brakes were (variably) poor, giving rear lockup on hard braking, until I cleared the pipes to the front.

Note that if the rears were not working due to a bad hose, there may be crap built up in the cylinders, which causes all sorts of baffling symptoms, until they start to leak.

FRM
FR Millmore

I'm with RJ. I have an very minor leak in an axle seal, and the smallest amount of oil causes the drum to lock. A quick shot of brake cleaner on the shoes and drum, followed by a wipedown with a rag, solves the problem for awhile. Another winter project to replace seals.

Ken
k v morton

This thread was discussed on 04/08/2009

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