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MG MGB Technical - Weak rear brakes

The rear brakes are weak on my 72 GT. I try bleeding the brakes and all I get is a very small squirt of fluid, with little pressure behind it. The brake hoses look fairly new, but looks could be deceiving.

The brake warning light also stays on all the time signifying an imbalance in pressure.

Front brakes are very strong.

Any help would be greatly appreciated, as always.

Thanks,
Dave
72 GT 258005
Dave Shively

The rear brakes do not give much braking effort, as under braking weight is moved forward and the rear wheels have a much greater tendency to lock than the fronts.

If the car has been trailered at some point some people lash the back axle with chains or straps and this can crush the pipes. But if you are getting fluid out of each rear slave bleed nipple then they should be clear enough, unless you are really having to stand on the pedal to force it out.

In what way are you detecting they are weak? If by the handbrake then again this is normal, it is only a parking brake and not an 'emergency' brake as many call it. If you define an emergency as needing to use your front brakes and finding they have failed, then pulling on the handbrake is nothing more than a gesture, like a cartoon character (other than a bird) flapping their arms when they fall off a cliff. But BL handbrakes were never very good, they need regular maintenance or they will become even weaker. By rolling mine down a sloping drive and leaning in and pulling up on the handbrake I can lock my rears easily, but not when I am sitting inside (and I ain't heavy by today's standards!). At each service I dismantle them, clean and lightly grease the handbrake pivot points on the diff and backplates, and put a light smear of grease on every metal-to-metal contact point. The handbrake levers inside the drums can also wear, if when operated the end of the bar that the cable clevis goes through is being pulled past parallel with the back-plate you are losing effectiveness, sometimes putting a blob of weld in the notches of the levers can restore it to parallel will help.

Paul Hunt

Hi:

In the old days , the tow trucks used a hook to tow the car by the rear end. Low fluid could mean a crushed brake line on the differential case. I have seen this on numerous MGB's.

Good Luck: Rich Boris
Rich Boris

Your rear brakes may just need to be adjusted.
R. L Carleen

If the brake warning light is on continuously that could mean that the piston in the pressure difference switch isn't centralised, switching the light, but also cutting off (most of) the flow to the rear brakes.
As a check, get an assistant to stand on the brakes (with the ignition on so the warning light is on) and open one of the bleed nipples of the front brakes a little. This could/should push the piston back to centre position and the light will go out. As soon as the light is out, close the bleed nipple. If you overdo it and the light comes on again after going out, you'll have to repeat the procedure, but now opening one of the rear nipples.
HTH,
Rufus
Rufus Pool

Out of 4 rear axles I have owned, two of them had crushed rear brake lines from being towed at some time.
The pressure failure switch used on chrome bumper cars does not shut off fluid flow when the light is on. It does indicate a pressure difference between the front and rear braking systems. If the lines to the rear were empty it may take some time to get all the air out and get a good fluid stream for the longest line.
Clifton
Clifton Gordon

Dave, does the warning light come on as soon as the ignition is switched on? If so, then a pressure imbalance was the event that made the light come on. The piston being off-centre is what is making it stay on.
There could off course be a problem with a crushed line, but it would need to be on both sides of the diff. for both rear brakes to be equally bad.
The pressure warning piston will affect both rear brakes as much.
I could be completely wrong on this, the lit warning light could be caused by electrical problems (it is an MGB...), but I have a very strong gut feeling that the piston is causing your problems.
Rufus
Rufus Pool

The GT's use a different slave cylinder than the roadster.
Check it out.
Safety Fast
Dwight
Dwight McCullough

Hi Dave
Check out the hose between the rear brake line and the pipe to the master cylinder it can swell up and either give you weak brakes or brakes that wont release
pat
patrick bailey

To answer Rufus - When I bleed the rear brakes the light goes off for a short time; that is until I drive the car again. The light remains off, but 2 hours later the light comes on again.

Answer to R.L. - The rear brakes are adjusted.

To all - this problem only occured after the rear brake drums were removed to replace the axle bearings and seals.

Dave
Dave Shively

Just a comment, I was working on my 74.5 "B" and noticed the rear brake tubes running over the rear axle were crushed. I replaced them but could never figure out how they got that way. Now I know, by towing. This site always amazes with the knowledge you can gain from it.
John

If the warning light comes on again after a drive, my first guess would be that something is wrong with the rear brakes.
Have you removed the drums again to check for leakage?
Rufus
Rufus Pool

If bleeding the rear brakes makes the light go off, then it comes on again after driving a bit, then I would say there is a problem on the *other* circuit, not the rear brake circuit. But I always thought that the tandem systems had different master cylinder sections for each front caliper, one of them also feeding both rear brakes. But the diagrams in my Haynes seem to show that the front brakes share one section and the rears the other. Is this the case? If so I can't really see the point because if one of the front pipes fail you are going to lose 90% of your braking effort anyway.
Paul Hunt

This thread was discussed between 26/01/2005 and 06/02/2005

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