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MG MGB Technical - Brake problems

Hi all, a cross Bumsore here!

The missus 71 BGT developed spongy brakes last week, needed pumping to get any pressure at all. The guy at the garage (who had the back axle off a month ago) said it was the brake cylinder so he changed it. Then when we went to pick it up it was still the same and he said now that the servo was past it's sell by date, so more expense and it is duly changed. This evening she goes back to the garage to pick the car up and lo and behold it is still the same but now he says it is something in the back that has seized (don't know what he was shut by the time I got home from work).
What exactly causes brakes to behave this way ie need pumping to get them to work? I always thought it was either air in the system or leaky seals. Any help would be much appreciated before I go around there tomorrow and punch the idiots lights out for telling my missus that the car was still safe to drive and sending her off with a flea in her ear.

And just to get back into character:
Good show! Damned good show!

Major Bumsore

The first thing I would check for is a leaking, internally, master cylinder. Ray

Apologies That brake cylinder that was changed was the master cylinder. I was a little mad earlier when I wrote that.
Major Bumsore

Hello Major,

Since the master and servo were just changed, I would suspect that not all of the air has been bled out.

Then again, if by spongy, do you mean that it needs to be pumped to get a good pedal, but does get a good pedal? If that is the case, the rear brake shoes could be out of adjustment, and the adjusters are prone to seizing.

I never had a B with power brakes, but I have a vague recollection that it is a remote servo, which involves another hydraulic cylinder. If that is the case, I can imagine a scenario where the servo cylinder goes bad, allows the pedal to drop too far, and ruins a master that would not have failed had it not been depressed past the old dirt lines that defined its normal travel.

This is a bit of conjecture here on my part. The only remote servo vehicle I have is a 46 Chevy 2-ton, and the failure mode of it's servo was to allow all the fluid to get sucked into the intake manifold, which did result in the pedal going to the floor.

Nevertheless, it seems poor etiquette on your mechanic's part to send your sweetie off with it even if it was a legitimate snowball-effect failure mode.

C R Huff

A slowly descending pedal with no external leaks is normally the tell tale of leaking seals in the master cylinder. The fluid just basically recycles around the seals.

Failed seals the pedal goes straight to the floor.

Pumping the pedal producing , eventually , a firm pedal is normally air in the system. The pedal normally stays firm.

Pumping the pedal and the firm pedal gradually falling to the floor is usually a leak in the system.

Hope that helps and I understand your frustration.

Personally I think the blaggard deserves a damn good thrashing !

Peter Thomas

If the guy thinks a servo could possibly cause this, he is not a mechanic. Losing your brakes' power assist makes the pedal go to the floor? That's beyond asinine - the guy is incapable of thinking through a very simple system. He may be great at changing parts but that's a heck of an expensive way to troubleshoot. Especially when he begins with the two most expensive parts. Literally!

If you're not losing fluid, the first thing to check is, ahem, the adjustment of the rear brakes :(

Yes, alas, the rear brakes are too far out of adjustment, the pedal will be limp most of the way, and it'll finally apply the fronts right near the end of the pedal travel, like barely off the floor. If it happened fairly suddenly, check for a bad adjuster or other toasted hardware in the rear brakes.

But before replacing any more parts, just get the system properly adjusted and possibly bled by someone with a clue. Possibly you. You wouldn't rip yourself off, would you? Now. The adjuster is a 1/4 inch square shaft on the inboard side of the brake, toward the top. Handbrake off. Tighten until the wheel won't turn, then back off a couple clicks till it does. A little drag is okay, long as you can still turn it by hand. Repeat on other side. Now how do your brakes work?

Sounds to me like he let air in at the back when he was working on the axle, get it bled by someone who knows what they are doing. I agree checking adjustment is worth trying as well. A failed servo is easy to check, just turn the engine on with the brakes applied if its working you will notice the pedal sink as the servo force kicks in.
Stan Best

First thing I would do is find another garage. That is a typical rip-off, just keep changing parts at the customers expense in the hope you land on it.

First make sure the rear brakes are correctly adjusted or you will always have a long pedal, but it still shouldn't be spongy or pump up.

They can be tricky to bleed, even with a constant pressure device like an EeziBleed. Using that on both my cars I can bleed way after all the bubbles stop coming out but still get a long and spongy pedal that pumps up, then goes long again a few seconds later. I have to get someone to press hard on the brake pedal while I rapidly open and close each caliper nipple in turn, and that always gets an extra lump of air out, after that it is fine.

A good servo will hold its vacuum for several operations of the brake pedal with the engine off. You have to 'empty' that vacuum by pumping the brakes several times with the engine off, and that *should* make the pedal stiffen up and won't go down as far. Then start the engine and it should go down further again with the same pressure. If there is no difference then the servo *is* inoperable, and with the remote servo you will barely notice it.
Paul Hunt 2


Though I am not positive, I am reasonably sure that some MGBs used a remote servo (booster). Because a remote servo has its own hydraulic cylinder, they can either bypass or dump the brake fluid and cause the pedal to go to the floor. I have had it happen, though not on an MG.

C R Huff

That can indeed happen, but it would go to the floor every time and not respond to pumping as the original poster reports. The master would also be empty.
Paul Hunt 2

Thanks all. We have refused to pay him until he fixes this and won't be paying him any more than he has already billed us. He is blaming this now on some spring being sized in the nearside rear drum.

The symptoms were brake pedal to the floor with no brakes at all and after each pump it would progressively get better and eventually be as good as you could expect. I can't say as I understand brake systems so to be honest I'm lost, I don't understand how a even a sized pad/shoe could result in that sort of behaviour; an air lock yes. One things for sure there is no way we will use this guy again.
Major Bumsore

CR don't think I ever saw a B with a remote booster unless it was aftermarket.Major,as you work on your brake system if you remember one simple rule it will all start to make sence, "Air is compressable, brake fluid is not" Ric

Try adjusting the rear brakes, assuming they are drums on your B. This worked on my son's Honda, and I'm not saying it will work here, but, hey, it costs nothing. Just tighten the rear drums by turning the star shaped wheel accessed from the back side of the wheel, or you can take the brake drum off, expand the brake pads slightly by turning the wheel, and putting the drum back on. Worked on the Honda, might work here.
1977 MGB
randy olson

Randy the honda system you discribe might confuse a MGB-Tech to be as it is a free floating system unlike the fixed shoe system on the B check out a good parts book to see how the brakes work ,RIC

This thread was discussed between 28/01/2008 and 05/02/2008

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