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MG Midget and Sprite Technical - Rebuilt brake caliper won't bleed

Replaced both front calipers on my 1971 midget with remanufactured ones from an inexpensive source. Now I'm bleeding the brakes and the left front one just barely bleeds at all. What would cause that?

Some background: replaced the two front rubber lines, as well as the back. Started by bleeding the left rear, then the right rear, then the right front, and all went great. Well on my way.

Then went to front left, and next to nothing happened. I figured the shuttle in the switch assembly must have shut down the front brakes, so I bled the rear. Still next to nothing at the front left. Bled the rear some more. No change. Tried the right front and it bleeds just fine, so now I suspect the caliper.

Removed the bleed screw entirely. The hole in the side of the screw looks clear, but I'm in the house now and didn't try blowing compressed air through it to be sure it's clear through. The lovely assistant SLOWLY depressed the pedal with the screw out, and fluid came out. Not sure if I can say it flowed as readily as it should, because it was a SLOW depression.

Not sure if it's helpful, but just a very small amount bleeds out of the left front. And the rotor turns fine, and I can't turn it by hand when the pedal is depressed. Clamps up tight.

I'm thinking of swapping the bleed screws just to see if the problem swaps, but I wonder if others have ever encountered this.

Do I misunderstand the function of the safety switch assembly mounted on the right fender? I thought its function was in the event of a leak in the front or rear lines, a shuttle moves to close off the leaking section (front OR rear, not one of each, right?) so the other section works. But my other front brake bleeds fine, so I think I moved the shuttle back enough. Is it actually one front and one rear in an X pattern?

One other thing I should mention. Just occurred to me: I have the switch from the safety assembly removed. I'll put it back in before doing anything else, but I wonder if that could be a factor. Still seems to me the other front brake shouldn't bleed if the switch assembly is the problem.

Could I have a poorly remanufactured caliper?

Thanks for any suggestions.
Mark 1275

Your understanding of the safety switch operation is correct
Front circuit and rear seperate

It's usually only some later front wheel drive vehicles that have the circuits crossed
These cars normally have large front brakes and tiny rears and need the circuit crossed so at least one front works if there's a failure as the rears on their own wouldn't hardly pull the car up without the fronts working

You could try removing the hose from the caliper and check the flow there--but if you have flow when your lovely pushed the pedal then it just about has to be a blocked bleeder

When it's working normally there's hardly any fluid actually flowing in the system it's more of a pressure thing, so if you have some flow with the bleeder out it should work
William Revit

First thin I'd do is swap the bleed screws, as you've already suggested.
Dave O'Neill 2

Just check you have the calipers on the right sides when i got mine the PO had fit them on the wrong sides so bleed nipple was at the bottom of the caliper so could never geta firm peddle.
mark heyworth

Thank you for the replies.

Next chance I get in the garage I'll swap the bleed screws and see what happens.

I do have the bleed screws up on both sides, so I'm okay there.
Mark 1275

Problem solved.

Swapped the bleeder screws, and the side with the problem had some metal bits in it. When I bled the side that was slow, more metal bits were visible coming out. Now it seems all clear and all good.

Thank you for the responses.

Now all I need is for it to be May. Even April.
Mark 1275


That would disturb me and make me want to return them as they shouldn't have metal debris in them as it could damage the seals, at least strip the calipers to make sure there was no more debris inside. It shouldn't happen but reminds me of the first Mocal oilstat I bought around 1987 and it came with nice red caps on the 4 ports, I dismantled it as I was curious about how it worked and was shocked to see quite a bit of swarf inside the oilstat, I cleaned it out and used it as all else seemed good but the presence of the swarf rang alarm bells about the companies QC arrangements.
David Billington

It's a real p%ssoff when new stuff comes with machining debris included
Last month I had a brand new oil pump for an MGB, it came in a sealed plastic bag and had some oil floating around in the bag--
Being one for not trusting anyone/anything unless I do it myself I usually pull everything apart and clean , check, and reassemble---just a me thing I do.
Because it was so clean/new and prelubed and in a sealed bag I 'nearly' fitted it as was but talked myself into checking it-
Pulled the pickup end off and pulled the gears out and out dropped a piece of alloy about the size of a grain of rice and paper thin--well well , where did you come from--
Checked it out and the hole through the pickup tube was almost completely blocked off where it entered the mounting end for the pump- there was a thin slither of casting flash blocking the hole all except the stray piece that fell out, knocked it out and cleaned the pickup hole out and fettled a nice curve into the plate, cleaned up and reassembled---could have been nasty, would have been quite an issue getting oil pressure up-or just getting oil up at all
Pain in the butt that things get sold half cooked
William Revit

It is a new policy of mine to dismantle and check everything as so many replacement parts, whether remanufactured or especially new are utter rubbish.
Clive Berry

This is what I got out of a new uprated rocker shaft from a major Mini supplier after tapping it for 10 minutes with the plastic handle of a screwdriver. I sent it back and got a refund including return postage.

MG Moneypit


Was it tufftrided or similar as IIRC Vizard mentions this problem and the need to pull the plugs and clean. I don't know if they're still available but the hardened rocker shaft I bought was apparently gas carbo nitrided and didn't suffer from the issue. I haven't had any issues with that shaft but when I've looked for them again I couldn't find any, I bought it in the late 1980s. I heard at the time that getting tufftriding done and similar processes was getting harder due to the chemicals used being much more tightly controlled, much like chrome plating.
David Billington

I bought a sliding hub for a gearbox rebuild, fitted it straight out of the box, completed the gearbox rebuild, and then installed gearbox and engine without further thought. And the gearbox wouldn't shift into second gear. Much swearing ensued. Took it all apart again and found that the hub had been assembled one tooth out of the correct position so that the mating part couldn't mate.... a day of extra work to rectify.

I now check EVERYTHING before I install it
Dominic Clancy

David Billington, it was "uprated" by being made of thicker material. The shaft was sealed at both ends by a pushed in plug so removing and cleaning was beyond me. Originals had screw in plugs but of course where savings can be made then they will, to the detriment of the end user.

I had a telephone conversation with one of the guys there and he did admit that they had a batch which had swarf contamination, they had sent the whole batch back to the supplier requesting they remove the swarf and return the batch. I was one of the first customers to buy from the returned batch. I suspect the supplier binned the original batch (too expensive to clean?) and just supplied a new batch.

MG Moneypit


Personally I've only ever seen the ones with pressed in plugs which I'm fairly sure were OE, maybe the early OE ones had screw in ones I don't know. I've not dug out Vizard but I seem to recall the need to remove the pressed in plugs and replace them after cleaning with similar plugs or tapping and fitting screwed plugs.
David Billington

My pet hate is Mazda crankshafts-
They are drilled through from the main journal to the bigend as normal but then cross drilled and then a ball bearing pressed into the end of the long hole, what a rubbish trap, the only way to be 100% sure everything is out is to drive the ball out and refit it after---here's the problem--is that tight enough-doesn't feel like it- will loctite hold it -don't risk it-
So you end up tapping the holes out and fitting screw in bungs, then have to get it rebalanced just to make sure--
William Revit

This thread was discussed between 14/02/2020 and 18/02/2020

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