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MG Midget and Sprite Technical - Brakes sticking on

It's not often I will admit I am really confused and stuck... but this has me flummoxed right now!

Finally got the car out the garage yesterday and managed a lap of the block shake down test. I parked in the garage, but after getting out I wanted it a bit further forward... Nope - brakes were locked on! I thought it felt a bit more sluggish than I remembered it! ha ha!

Before I forget - 1500 with single circuit brakes.

Back jacked up, the back wheels turned OK.

Front jacked up, both wheels required huge effort to rotate. Both fronts locked on.

Decided to investigate passenger side first, just because it was easiest to get to. Caliper off, jumped in the car and pressed the pedal, foot goes to the floor and pistons in the caliper move.

I try to retract the pistons. Pushing one in and the other pops out. Try pushing both in, locked solid.

I open the bleed nipple and use my vacuum bleeder, nothing much happens.

I take the caliper off, on the bench the pistons seem to move OK, so I think the caliper is fine.

I put the hose in a tub and press the brake, lots of fluid scooshes out, so not a (total) blockage.

At some point along the line the drivers side caliper has freed itself and that wheel now turns OK.

Reassemble everything, bleed passenger side. Pump up the pedal and it's all locked up again (both sides).

It's like something is acting as a one way valve. But... if the master cylinder was faulty the rear brakes would have locked too, they seemed OK.

If there is a fault on the passenger side, why does the driver side also lock?

If there is a fault on both front sides, why when the pressure is relieved on the passenger side, does the drivers side then unbind?

Thanks for reading, if you got this far! If anyone can figure out whats going on, the BBS collective can!

Cheers,
Malc.
Malcolm

I would check the rubber hoses. One or both could have some internal collapsing. I had that problem once with the rear hose going from the solid line to the rear axle. Replaced the hose and the problem went away.
Martin

Thanks Martin, should have said that they are "fancy" braided hoses. Yes, they could still fail, but I will be p-ed off if they have!

Malc.
Malcolm

Malcolm,
I agree with Martin, I had similar problem with rear hose perishing and causing rear brakes to bind. Replaced hose and all good.
While it would be coincidental for both front hoses to fail, the symptoms sound very similar, so I would change both the front hoses.
Simon
S Holt

Malcolm,
Guess my post overlapped with yours, noted braided hoses fitted, so unlikely to be the problem!
Simon
S Holt

Malcolm, at least some of the "fancy" braided hoses still have a rubber internal pipe within the SS braiding so I guess can still collapse and form the one-way valve syndrome. Did you try pumping fluid through them the wrong way (would need to rig something up to do this)
GuyW

If this was an earlier car with the tandom MC I would say your brake push rod was too long.
Rob
MG Moneypit

Malc.
Bit strange that releasing one side releases both--
You'll have to start at the calipers and work your way up--pump the brakes up--and release the pedal
If the fronts are bound up then undo the front bleeders , if that releases the brakes, then the calipers are ok---pump it up again, this time release the pressure at the other end of the flex hoses, if the brakes release then the hoses are ok ,if not the hoses are faulty---if it was ok there, the next spot would be the manifold where the front pipes join, unlikely but you never know---it'll just be a process of elimination-
willy
William Revit

I hadn't spotted that releasing pressure on one front also eased the other. With the feeds separate back as far as the 4 way junction this rather suggests a m/cyl fault. But then why are the fronts locking and the rears releasing? Unless, does it have a rear brake line bias valve? Perhaps the rears are never locking up that tight in the first place so when the m/cyl won't release pressure it appears that the rears have released.
GuyW

Thanks for the input everyone. Not had further chance to investigate as I am also elbow deep in my Alfa (too many projects going on right now!). I might order some new hoses as I can do that from the comfort of my office chair.

Malc.
Malcolm

Oh no not elbow deep in an Alfa--Got your tetanus shots up to date---LOL
William Revit

Malc

Here follows some daft questions and odd ramblings.

How worn down are your front brake pads? I wonder what the condition of the caliper piston bores and seals are like?

Probably front brake hoses shot as others say.

Have you flushed system with new fluid and re-bled it?

If your front flexi hoses are under suspicion, I wonder if the flexi at the rear is OK? Did the rear brakes work fine? I am wondering if there is a blockage at the rear flexi which may have over favoured the braking to the front on a single circuit system more so than normal.

Cheers
Mike
M Wood

Malc. Interesting problem you are trying to diagnose. I have no experience with the home market cars, only the North American specification vehicles. Hence, my thoughts may be off base.

As you note, with the old style, single chamber, master cylinder both the front and rear brakes are operated by fluid through a single orifice on the master cylinder. The fact that the rear brakes function properly would seem to indicate that the master cylinder is, also, functioning properly.

Where does the line from the master cylinder go next? On my 68 BGT, the line goes from the master cylinder to a four way connection (MC, rear brakes, right front and left front). This is the first place where the two front brake systems could be both influenced by a single piece of hardware. Might be worth, if you have it, removing, inspecting, and cleaning the connector.

The flex lines having the stainless steel braided covering, at one time, had a bad reputation and people found they failed more commonly than the non-covered lines. Whether that is still true, I do not know. But, worth checking. Removing the lines and trying to flow compressed air through them from the caliper side to the hard line side might give you an idea if they have failed internally. I have not tried that myself.

Just some ideas, but you have a most interesting problem which, I am sure, all of us will have something to learn from. Good luck.

Les
Les Bengtson

Malc, looking at your symptoms at first they seem contradictory. What if the master cylinder was maintaining a small amount of pressure in the system, the pistons in rear cylinders are quite small and are retracted by springs, the calipers having larger pistons and no return springs could possibly be held out without the rears being affected.

Bob
R.A Davis

Thanks again everyone.

Pads have plenty left on them. All worked well when I last drove it last summer. The passenger side caliper seemed OK on the bench. It has only been 12 months since they were off the car and cleaned up.

Three things are now making me think it is a M/C problem.

1. The split for the drivers front is right at the master cylinder, this setup:

https://www.moss-europe.co.uk/shop-by-model/mg/midget/brakes/brake-hydraulics-controls/brake-pipes-single-line-system-midget-1500-1974-79.html

If both fronts are locking, the M/C is the only common component before the split.

2. I am struggling to believe that both hoses would have failed at the same time. They are "proper" branded Goodridge PTFE lined/stainless braided. They should outlast the car.

3. What Bob said, the rears not locking could be a red herring. There could be enough force in the springs to pull the back just enough.

Malc.
Malcolm

Malc

Why not give Powertrack braking a call to see how much a genuine AP Caparo (I forget exactly what they are called now - ‘920 Engineering’ IIRC) master cylinder is and availability of them would be? https://www.powertrackbrakes.co.uk/

I know throwing new bits at a problem before diagnosis is not ideal.

Checking the brass union piece for blockage - worth checking as suggested earlier? I would also make sure the flexi brake pipe to the rear axle is OK.

As for your braided hoses - wonder what the life of the rubber in them these days is? It is one things I do not like about them - less exterior visual clues on possible internal condition. How old are they?

Cheers
Mike
M Wood

Hi Mike, I don't know excatly how old they are off the top of my head, but IIRC I fitted them when the car was in my current garage, so less than five years ago.

If I get time tonight I am going to remove the master.

Malc.
Malcolm

Mike

I've used Goodridge hoses for more than 10 years and always found them to be excellent quality. The inner lining is PTFE not rubber and although I'm sure this doesn't make them immune to degradation I think it has a better chance than rubber. My main reason for fitting them is they give a firmer brake pedal.

Bob
R.A Davis

I fitted my braided Goodridge hoses in 1995!. Thinking they were potentially dangerous I changed them 3 years ago for a new set. On examining the old ones and cutting one in half I could see no signs of breakdown or damage. The PTFE lining was in excellent condition and no evidence of corrosion or weakness at the joint ends. That said replacement were not that expensive and it gave me peace of mind! I could not find any recommendations when these hoses should be renewed but that of course does not mean there isn't any!

Bob Beaumont

Bob, mine are from much the same time. Only I haven't replaced them yet. Perhaps I should!

There was MASC article quite a few years ago about braided hoses bursting apart where the PTFE lining fitted into the ends, but that may have been under racing conditions. I checked mine then but couldn't see any deterioration.
GuyW

Guy, some people make up their own type of 'Aeroquip'(?) hoses so possibly not up to factory manufacture conditions and specifications. My understanding (so may well be totally wrong) is that you inspect them and replace as required, there's no stated life that I've seen.
Nigel Atkins

Managed to liberate the master cylinder last night. What a 'mare!

As you all probably know, you have to remove the clutch cylinder first, no real bother as it has a flexi pipe so just can undo and move out the way. Just a bit of a faff.

Then the brake M/C... the brake line that goes into the master was seized solid. Nut got completly mashed and I ended up having to cut the brake line. Urgh! Another job!

M/C apart, a bit gungy but nothing obviously wrong... although I wasn't expecting much, it's not exactly a complex bit of kit!

I will give it a thorough cleaning and rebuild and see where we go from there.

Might as well pull the pedal box now too and give that a lick of paint.

Cheers,
Malc.
Malcolm

"I try to retract the pistons. Pushing one in and the other pops out. Try pushing both in, locked solid.

I open the bleed nipple and use my vacuum bleeder, nothing much happens."

---

If you can't expel fluid via the bleed nipple, it sounds to me that a passage is at least partially blocked inside the caliper, trapping fluid behind the pistons. Fluid can be pushed in, passed that blockage, but there is insufficient pressure, to push the fluid back passed the 'blockage', in the reverse direction.

I think with all you've alrrady done, I'd split the caliper(s) for a thorough clean and inspection. Or at least fully pop out the pistons.
anamnesis

Good job Malc. While the pedal box is out you could check the condition of brake and clutch pedal's pivot bolt, do as Guy did and put in a couple of grease nipples to stop it seizing in future?
While you're there maybe you could tidy up the paintwork in that part of the engine bay or the wiring - now what was the original job again?
Jeremy MkIII

Stop it Jeremy! I am already bad enough for doing that without your encouragement. :-D
Malcolm

Sorry Malc!
Guy's pivot pedal greasing idea is a good one though as you're unlikely to remove the pedal box again any time soon. I did it on mine and grease it when I remember.
Jeremy MkIII

Malc

Did you sort this problem? If so, what did you do and what worked?

Another Chevalier Classics fix it kit in prep perhaps?

Cheers
Mike

M Wood

Hi Mike,

Been away so no progress really. Cleaned, inspected and rebuilt the M/C, it seemed OK, just a bit gungy. Dug out the spare I had and did the same to that one too.

Pedal box has had a lick of paint, will try to get it reassembled this weekend and see how we are doing.

Cheers,
Malc.
Malcolm

This thread was discussed between 26/02/2022 and 18/03/2022

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