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MG MGB Technical - Spongy brakes

I look after a friend's pull handle B as well as my own MGA. It has a recurring problem with spongy brakes, which has now got me stumped.

1. I have bled the brakes each year and the problem always returns
2. master cylinder is new (old one DID leak
3. I find no leaks when putting the system under pressure and holding the pressure for many minutes
4. All flex hoses have been replaced
5. All cylinder at the wheels appear to be in order
6. Rear drums are adjusted correctly
7. No leaks at the MC banjos
8, MC fluid level appears unchanged, cylinder is full from last service without being topped up
9, Brakes are hard after bleeding, but soften up after a few weeks
10 Symptoms are like air in the lines, but when I bleed, there is no sign of air being expelled.

So why do the brakes keep going spongy, and what can I do?

I am going to replace the rear hub seals this week, and fit new rear shoes, so as I will have everything apart, it would be great to have the tips to fix this permanently.
dominic clancy

You don't say whether the car is fitted with a servo.
It was the servo and the transverse, bulkhead lines which caused bleeding problems. At the factory the lines were vacuum bled. But later, when you try to bleed conventionally, trapped air couldn't be evacuated. If you have a servo, remove the front "stay" and lift up the front of the servo as far as it will go, then bleed with the engine ticking over. If you have a pressure or vacuum bleeder, use it.
Allan

No servo
dominic clancy

And I use an Eezibleed
dominic clancy

i had this problem and it was the front disc's worn,
put new one's on never had it again,
you can get a long pedel with worn bearing,
da wright

If the brakes harden up after bleeding, then go spongy after a period of time, then go hard again after re-bleeding and so on, then there must be something getting into the hydralics. It can only be air or some other gas in order to be compressible, regardless of not being able to see air in the bled fluid. If it were a mechanical problem with bearings or discs I wouldn't expect bleeding to make it better for a period of time. You can eliminate that and anything in the calipers and rear drums as a cause by doing the test before and after clamping off both front and the rear hoses.
Paul Hunt

When the pedal goes soft, does it improve with pumping? i.e does it get firmer?
As experiments try what Paul suggested, i.e. clampin g off the flexibles when the pedal is soft and see if it improves. You can also try wedging a suitable brace between the pedal and seat, which puts the system undetr prassure. Leave it over night and see if the brace is stiil tight the next morning. If it has it's either a leak or you have a faulty M/C..........not unknown!!
Allan

I have replaced the rear axle oil seals tonight (the old ones were rock hard, one snapped as I removed it), and fitted new brake shoes which I have adjusted well up to the drums. I am going to take it for a spin in a few minutes and see whether that has hardened the pedal up, as I have not yet bled the brakes again. Looking at the brake pipes on the car, I think I am going to replace these before I give the car back to the owner . They have certainly seen better days, and if I don't do them now, I'll probably have to do them next summer anyway, I'll keeo you all posted.

BTW, rear brakes on a B are a real pain to put back together compared tp an A. That handbrake lever assembly is insanely stubborn!
dominic clancy

The tool that Alan mentioned are very easy to make and a very useful tool, all you need is a 1 metre length of 15 mm tubing and a piece of flat steel bar approx 50mm x 150-200mm x 3mm thick, just drill a hole near one end of the flat bar, the hole need a slight clearance so it will work. To use it you put the tube end onto the pedal and slide the flat bat upto the seat cushion, press on the tube and the flat bar in opposite directions and the flat bar will lock onto the tube.
Dominic, the back brakes on a "B" are pretty straight forward, if you assemble the shoes onto the backplate,with the bottom springs fitted, but leave off the top spring until you have fitted the shoe steady springs, then you can put on the top spring using a pair of vise grips to grip just behind the hooked end of the spring and pull the spring into place.
john wright

Are you using silicon fluid?? aA chemist from MOTUL told us when we had a talk by at our workshop that silicon fluid always has air bubbles along the lines which then goes to the highest point. This problem can be reduced by pressure bleeding. After rebuilding my brakes on my GT I used silicon fluid & then went racing on an air field circuit. After 3 laps I had complete brake failure & ran off at a very convenient place. I syphoned it all out & refilled with normal fluid & had no further problems. Talking to others after the meet I found 2 guys that had had a similar experience.
To find out where the spongyness comes I clamp off all the hoses to see if it still there. If it is then the M/C is the problem. The brake pipe goes up from the M/C so air can be trapped there. If the pedal is solid then release one hose at a time to see when the spongyness returns.
Garth
Garth Bagnall

What Garth says is basically why I have to high-pressure bleed the fronts after doing any work on the hydraulics, and having bled normally with an Eezibleed for example. The high-pressure bleeding consists of getting someone to press down on the pedal as hard as they can while I rapidly open and close each caliper nipple in turn, and always gets an extra lump of air out. However that is to correct a long pedal that pumps up, then goes long again if released for a few moments. If the pedal is initially correct, then goes spongy over a few days or so as seems to be happening with Dominic , that is something different i.e. air getting *in* to the system over time.

As for fitting the shoes I've never found a problem with the top spring. I assemble the shoes and the inner bottom spring, locate the bottoms of the shoes onto the handbrake lever and slave piston, fit the top spring behind the shoes and locate them on the adjuster. It's the bottom outer spring that takes a bit more effort, stretching it enough to get the tip of the 'second' end in the hole in the handbrake lever before pressing it down to click into place. Whether one puts the steady springs on before or after this is immaterial, but I don't see how you can put the top spring on *after* fitting these, behind the shoes where they should be at any rate.
Paul Hunt

In thirty years of repairing MGB's I reckon that 1 in 3 cars that came in for the first time had the rear brakes incorrectly assembled. To get it right - look at the brake you are working on & imagine that the lining gets dragged to the end of the shoe by the wheels forward rotation.
Garth
Garth Bagnall

Adjusting the rear shoes right up to the drums seems to have solved the problem, as the pedal is now firm without having re-bled. I'lll wait for the new shoes to bed in and then replace the aged pipes and bleed again, and think that will probably solve the issue once and for all.
dominic clancy

This thread was discussed between 06/10/2008 and 11/10/2008

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