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MG MGB Technical - Seized rear brakes

Well after sitting in AZ for the past year and a half I finally had my C shipped to NY. I’ve put over 100 miles on it since I got here. After going out for a drive this morning both of my rear brakes engaged and wouldn’t release. Since its both wheels It has me extra concerned. What would cause both to seize at once?

Alright they have now cooled and released. But my brake pedal is soft, and even after I pumping it goes right to the floor after a moment or two. I checked fluid levels and for leaks, everything looks good.
Thoughts?
Todds 69C

sounds like you have a couple of faults needing further investigation.
I'm not fully aware of the C set up fir the USA but I'm guessing you have a single line system with a servo?
If you are not losing fluid the soft pedal suggests the master cylinder is allowing the fluid to pass the seal back to the reservoir.
The locked up rears could be a servo problem or the flexi pipe at the rear could be breaking down internally preventing the fluid flow back along the line when the brake pedal is released.
If you haven't done a brake system overhaul recently now would be a good time to renew all the rubber components and fluid.

good luck
MGmike


M McAndrew

I would check the condition of the brake hoses (one at each front wheel, and one near the rear axle). Have an assistant pump the brake pedal while you check the hose for ballooning.
Brian Shaw

The rear brakes on my brother's B were staying on. The PO had backed off the adjusters in a vain attempt to cure the problem.

A new flexi hose fixed it.
Dave O'Neill2

The brake hoses can collapse internally. They will look OK on the outside.

The brake pressure will flow through the collapsed hose, but it can't return. It will start to act as a one-way valve, seizing the brakes.
Willem vd Veer

All of the above. All the rubber is probably perished. While replacing the seals and flexible you will also change the fluid and therefore get rid of the water! Also on the assumption you used the hand/park brake while out, check the compensator on the axle, is free to move.
Allan Reeling

It is actually a dual servo system. As for the soft pedal, it was firm while the brakes were hanging up. It was only after I drove it further and they overheated did the pedal go soft.
I did inspect all the lines again last night and recall replacing the front and rear flex lines about 12 years ago. Scary how time fly’s! Would you think I’m due again?
Todds 69C

Okay, "dual servo system" does that translate to a dual circuit, single servo or two servos? If the former, how's the split done eg back and front or across the corners?
If the soft pedal is present after the over heating it could be water in the fluid boiling out of the rear wheel cyls. Has it gone back to normal after cooling off?
12 years is not excessive for the flex lines but given the relatively low cost I would change them for the peace of mind. The fluid will be well past it's best if that's not been changed. Current thinking is to replace the fluid every few years but I think 5 or 6 is the upper limit for safe operation.

MGmike
M McAndrew

It has been couple of weeks since the last posting on this thread so the problem being discussed may have been found and corrected. Just in case this has not happened, I would like to add a comment on my own expernence. When I bought my 1972 MGB a dozen years ago, I also had a problem with the brakes locking up. I don't know whether it was the front or back brakes, and it didn't matter. The car was immovable for 45 minutes to an hour after the brakes were applied. The problem turned out to be the proportioning valve. The brake fluid in it had become something thicker than molasses. Under the pressure of the brake pedal it would let fluid through to the wheels, but it would only let the pressure off over a long period of time. If your problem is with the brakes on at least two of your wheels, the proportioning valve may be a contributing factor.

Good luck!
glq Greg

I would start with the Master Cylinder. If both systems are locked up, probability is that the seals in the Master are gone and are plugging the return hole in the master. This was a common issue in the B system with the brake switch on the clutch/brake cover. Screw it in a tad too much and the master will not recover enough to allow fluid to get back in and the brakes lock up.
Bruce Cunha

Check the brake lines on the top of the rear axle. When these cars are shipped, it is a common practice to tie down the rear axle with hold downs that go around the axle. In doing so, they often crush the brake lines causing the problem that you have described. RAY
rjm RAY

As Ray says it is advisable to check your brake pipes. On my MkI I couldn't get a bleed on one of the brakes and it wasn't really operating. I did new hoses, etc. Finally I found that the brake line (which was original aluminum) had been crushed in one spot. After installing a replacement all was well.

Robert McCoy

"(which was original aluminum) "

It was NOT! The lines were always terne coated steel. I have never seen an aluminum brake line in any car ever, despite the 5 million times I heard this, and I have changed enough lines to sink a big boat.

FRM
FR Millmore

FRM,
You're right. All the other lines which are original are coated steel. My mind must have spun an aluminum tubing story. I think aluminum would be a poor choice for this application.

In any event it was 95% closed due to a crimp on top of the axle. I don't think a chain did the damage. Almost like someone hit it with a tool. In any event a new pipe did the trick (along with a new flex hose just to be sure).

Bob
Robert McCoy

Bob -
The aluminum brake pipe story has been around at least since I started doing this full time, c1965. I once saw something that led me to believe that some Mercedes actually did have aluminum tubes, but haven't ever seen one. Knowing Mercedes, it is probably actually a krypto-vulcan cantgettium alloy!

The crushed tubes from tow truck morons are unbelievably common - I have replaced the same pipe on the same cars several times - I think the record was 4x on both sides of a B. Generally the damage looks like it was hit with a hammer or a round bar (the hooks now generally used on tilt beds).

FRM
FR Millmore

This thread was discussed between 12/08/2012 and 07/09/2012

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