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AndyW

Brake lock-up

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Lately I'm finding that the left front wheel on my Superspec is very prone to locking up with only moderate pedal application. That's on a good dry road; on loose surfaces or wet roads I have to be SO careful not to lose the front end under braking.

What would make one disc brake more sensitive than the other side? It seems much worse than it used to, so what should I be checking, cleaning or replacing to get more equal and progressive front brake performance and stop the lock-ups?

Brakes are the normal Superspec Sierra vented discs at front and drums rear, with standard pads and shoes that have only done 6k miles so plenty of meat left on them, although everything is now 9 years old.

Andy

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It's a matter of going through everything on the brakes methodically.

First thing is you mention all the brakes are 9 years old now.

Have you changed the brake fluid? This must be done at least every 3 years unless using 5.1 fluid, reason is that most brake fluids absorb water and this can convert to steam in hot brakes, resulting in air locks.

Secondly at 9 years the brake discs/drums could have built up rust in the operating cylinders which you will not be able to see unless disassembled making the operation less than smooth and ultimately locking components.

At 9 years I would personally give the whole system a full check up, removing brake pads, checking cylinders, checking pipe runs, reassembling with copper grease where required, fully bleeding through with new fluid.

If all this fails you may have a failing wheel cylinder, however this usually shows up by having fluid leak from it.

Lastly you mention that you have only done 6000 miles presumably in the 9 years, this low usage will often show up in moving parts seizing or getting stiff. An annual service for all motor vehicles is essential to maintain good reliability.

Jim

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Jim has it right, check everything after that much time.   I had the same thing happen on my Jag and it turned out to be the rubber hose connecting the rear hard pipes.  Turns out it had swelled internally and was acting as a check valve.   

Steve

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Make sure there is free play on the MC operating rod, when the brake pedal is released.

Make sure the piston in the right hand caliper is free. If its not working 100% it would feel like the LHS is more sensitive

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Great feedback thanks Jim, Steve & Bob.

I'm not sure if or when the previous owner changed the brake fluid so I was going to do that anyway. And as far as I can see there are no leaks from the system. But I'll give the brakes a complete strip down and clean and see if anything looks amiss with the pistons in the calipers.

BTW, does anyone know what size/type the unions are on the Superspec Sierra front brake hoses? I might as well change to some nice new braided hoses while I'm stripping the brakes down, but not sure of the fittings. There are banjos on the calipers and some sort of bulkhead bracket/joiner on the chassis rail to the copper pipe.

 

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When I took mine to Richard at GBS to have the scuttle redone he asked me a couple of superspec related questions like the shocks being squeaky and the brakes locking up.

 

He said that they couldn't figure out why it would lock one wheel or both at random times. They even tried different brake servos but couldn't get the braking to be even unless it was with the one fitted to it. 

In short it may be a superspec bug (one of those things that were left for us to fix) as I have it on the right front brake

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Certainly agree that squeaky shocks is pretty standard for the Superspec (if you still have Zimmers fitted), but I am not aware of any brake problems.   I have locked them a few times but only when heavy braking on wet/gravel roads.

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I’ve now fully stripped and checked over my brakes - removed and cleaned the front callipers and pads, removed rear drums and shoes, cleaned and reset the adjuster mechanisms, put copper grease on all contact points, and changed the brake fluid and bled the system.

The discs and drums were not rusty or scored, the pads and shoes were all in good condition with plenty of material left, and all the brake cylinder seals looked good with no leaks, although I didn’t check that the cylinders were free to move. All the brake pipes, unions and rubber hoses also looked good.

A test drive afterwards showed there was little or no improvement to my original issue. The left front brake still locks under medium-heavy braking.

Oddly the brake shoes on the right rear had hardly any wear at all as they still had sanding marks on the surfaces presumably from when the last owner fitted them. So it looks as if the right rear brake has not been working much (or at all?). Would this have any affect on the diagonally opposite front brake?

Any other ideas I can check?.

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Is the shoe auto adjuster free moving and setting in new position when released. The ratchet teeth on them wear/corrode sometimes and don't 'set' the new position. Don't see why that would affect the other brakes apart from a slightly longer pedal.

Does the front left come direct from the master cylinder or share with other lines?

Have you checked run out of the left front disc itself or its thickness at intervals round the rim? Either of these could cause a snatch while rotating leading to locking up. I would be tempted by a new pair of discs and pads if there is doubt.

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Have to admit that I found the shoe adjuster on that wheel looked to have jammed. The ratchet teeth on the cam were filled with something like paint (not rust) which was stopping it moving. Cleaned it up and it now moves ok, so maybe I’ve rectified that issue.

All four wheels have their own brake pipe from the master cylinder, so the locking wheel isn’t shared.

Is there an easy way to check disc run out as I don’t have a gauge? I can check the thickness at various places round the rim, although the discs and pads have only done 6k miles so shouldn’t really be worn.

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I’m doing my brakes as we speak, found the left front also locks first but not by much. The master cylinder has 3 connections front left, front right and rear, once tyres and brakes are warm the front pulls up square I think the 1 wheel first thing is a Robin Hood trait.

The rear drums are not worn much at all, this is normal for drum brakes on a light car and remember at IVA they check the tears don’t lock up first this balance is not easy to adjust but you don’t need more braking to the rear in normal driving

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I would do this with the wheel off but the nuts back on and nipped up.A dial gauge on a heavy stand is the standard method for checking runout. Gives you an actual figure and a nice visual representation as the needle moves too and fro if the disc is warped. Failing that make a rigid frame from wood with a big nail sticking out to touch the disc face and weight it down ++.  You thus have a fixed point sitting say a foot off the ground. Move the frame till the tip of the pointer is a couple of thou away from the disc face. Get your best glasses on and watch for an opening and closing gap between pointer and disc as you rotate the disc. If it remains in contact all the way round tap the frame further from the disc a few thou and spin again. Careful observation and you should see a tiny gap which ideally remains constant as you spin the disc. You could use feeler gauges at closest and furthest gap to get an idea of runout. Bit Heath Robinson but would give you an idea.

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I had one front wheel locking up on braking on my 2b. Turned out there was a slight bulge in my tyre due to the car standing over winter. I jacked the wheel a couple of ml off the floor and rotated and you could then see it

Andi

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Solved the issue with the lack of wear on the rear brake shoes. 3 of the 4 pistons were seized in the cylinders so I had no rear braking effort at all! Surprised to find that my local motor factor had some Sierra drum brake cylinders on the shelf, so a quick replacement and that’s fixed.

Now to get back to investigating the front disc lockup. 

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