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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Try more free play adjustment in the pedal. ie slacken any adjuster off.
Steve.

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?

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Originally posted by robinj66
I have also reconditioned my master cylinder.

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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.

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

Just to save the hassle it may be worth getting a new master cylinder. They're not that much are they?

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]

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

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!!

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...

Robin I have the lucas instructions for the Sierra Mc. could copy it and send to you if it is of use.

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.