robinj66
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 posted on 17/11/06 at 03:25 PM |
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Brake Problems
This relates to my Cobra replica.
I have recently changed my standard Cortina solid discs and M16 calipers. I have replaced them with vented capri discs and Princess 4 pot calipers.
Sounds lovely so far...
Discs were brand new and fit the Cortina stub axle ; the calipers were secondhand but in good condition with all 4 pistons in each unit moving freely.
They have an 8mm spacer. Brand new pads have been fitted.
This is the same setup as the guy who sold them to me - he had them fitted on a rally car.
I have also reconditioned my master cylinder.
My problem is that, once bled the pedal is quite hard and at the top of its travel - sounds great except once they are hot the brakes lock on - very
embarrassing, blue discs and smoke from the brakes, stuck solid at the side of the road.
Once a bleed nipple is opened on each caliper and fluid ejected, the brakes nolonger lock on but the pedal is slightly spongy. Hence brakes are all
bled again - road test and brakes locking once hot ( as before).
Done this three times and now at a loss to understand why this is happening and what to do about it. Any words of wisdom?
For the avoidance of doubt the master cylinder has been pulled apart and my work checked by a third party. Pads have been changed.
__________________
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stevec
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| posted on 17/11/06 at 03:33 PM |
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Try more free play adjustment in the pedal. ie slacken any adjuster off.
Steve.
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RazMan
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| posted on 17/11/06 at 04:16 PM |
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Sound almost as if the fluid is boiling in the calipers and creating back pressure. Is the fluid fresh? Also, is the m/c coming all the way back - ie
not obstructed by pedal travel?
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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02GF74
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| posted on 17/11/06 at 04:36 PM |
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quote:
Originally posted by robinj66
I have also reconditioned my master cylinder.
my reaction to that is 
ARe you 100% sure you assembled it correctly? It is so easy to get one part wrong...
Ok, jack up the fornt wheels, can you turn them freely by hand? The pads may drag a little bit but not to stop them turning.,
Now applya brake and relase, the wheels should turn with same resistance, if not, smething is wrong.
Test both sides.
Is it possible that disc ti too thick ofr the calipers, can't see how it can be but something to rule out.
Are piston seals fitted correctly? the rubber seals act to pull the pistons back in - I cannot see how you can fit them so this doesn't work
but ......
dunno what else to suggest (my money is on problem with master).
why was the guy selling htese bits - di he have the same probem with them? (look for signs of burnt off paint  )
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DIY Si
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| posted on 17/11/06 at 04:43 PM |
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What sort of flexi pipes have you got? If old(ish) rubber ones, they could be blocking the fluid and not allowing it to return to the master. This
would then slowly creep back to normal. May not be this, but either way the fluid isn't returning to/through the master.
“Let your plans be dark and as impenetratable as night, and when you move, fall like a thunderbolt.”
Sun Tzu, The Art of War
My new blog: http://spritecave.blogspot.co.uk/
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robinj66
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| posted on 17/11/06 at 06:12 PM |
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Fluid is fresh (DOT 4) and so are the rubber pipes (but good thinking).
Discs do fit within the calipers & when new pads were put in there was plenty of room to wiggle them.
The brake pedal does not stop the mastercylinder returning. However
I am coming round to the idea that the master cylinder must be at fault. For one thing, the fault appears to be on both wheels.I only had access to
haynes manuals which are not the clear as to which way round the seals on the primary/secondary piston go - anyone got a diagram from a Ford workshop
Manual?
Will follow your advice 02GF74 and see if that sheds any light
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DIY Si
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| posted on 17/11/06 at 06:21 PM |
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Just to save the hassle it may be worth getting a new master cylinder. They're not that much are they?
“Let your plans be dark and as impenetratable as night, and when you move, fall like a thunderbolt.”
Sun Tzu, The Art of War
My new blog: http://spritecave.blogspot.co.uk/
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Stu16v
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| posted on 17/11/06 at 07:01 PM |
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Almost definately nothing to do with the calipers. If you get it to the point of slowing down on its own accord again, try loosening the front brake
pipe circuit union at the master cylinder. If pressure bleeds off, and the car rolls again, the problem lies at the M/C - try ensuring that the brake
pedal has 'free' travel first and foremost. If loosening the union doesnt make a difference, do it again, but try releasing pressure at
the brake pipe to flexi lines, and so on. Methinks you wont get that far though...
[Edited on 17/11/06 by Stu16v]
Dont just build it.....make it!
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mark chandler
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| posted on 17/11/06 at 07:50 PM |
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Sound like the master cylinder stroke is not returning fully to the rest position, ie the fluid hole that feeds the cylinder should be infron of the
seals.
Either adjust the pedal so the rod has more travel or shorten the rod, either way the starting point within the cylinder needs to be earlier.
Strange if it worked okay before though as moving more or less fluid should not affect anything unless the quantity is much larger.
Regards Mark
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robinj66
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| posted on 17/11/06 at 09:46 PM |
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Thanks
Consensus seems to be the problem lies with the master cylinder. Gonna strip it (again) tomorrow and check the relative positions of the seals (ie,
whether they face towards or away from each other.
Trying to find a new master cylinder is more of a problem than the cost, although Partco quote around £90!!
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jambojeef
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| posted on 17/11/06 at 10:39 PM |
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I had this exact problem on a motorbike a few years ago - brakes were slowly locking the front wheel - had no idea what it was but thought i was
losing power - changed down, gave it some gas - front wheel washed out and threw me off.
Not a nice tale but that turned out to be the master cylinder - so I would go along with the general felling from other posts - take it so carefully
when you're checking it though...
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Dingz
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| posted on 17/11/06 at 11:34 PM |
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Robin I have the lucas instructions for the Sierra Mc. could copy it and send to you if it is of use.
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02GF74
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| posted on 18/11/06 at 09:44 PM |
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I bought a new sierra master, about £ 70 from allparts and then was pointed to a website that lists them for about £ 40; If you search for my post
on this topic, I am sure it will come up.
I also have plathora of Haynes for Fords - you you know model/year, then I'll be able to do a scan for ya on Monday.
My experience cleaning a clutch master apart on a Land Rover is that it all looks obvious when yo utalke it apart, when you come to fit it together is
is not so obvious and certainl ywth the seals, they can be fit on the pistons in 2 ways - so best to take photo/drawing as you take it apart.
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