Brake Problems

[robinj66]

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.

[stevee8]

Robin,
I think I am correct in saying that these calipers require two pipe connections - presumably you have this OK? I am assuming that only one connection will somehow compromise the performance / operation.

The spacer is presumably to align the caliper to the disc? Have you double checked this?

You say that the pistons are free - has this been checked over the whole travel? I'm just thinking that sometimes they can stick at certain points, especially extents of travel.

[wilf]

I think that you might look at your brake pedal - if it is not releasing totally the master cylinder, you can get effects like this. Try a stronger pull off spring or adjust the clevis to give a little more free play or "slack movement" - i.e. shorten the pushrod.

[stevee8]

Servo problem can also have a similar effect - might be worth temporary disconnection to try it.

[robinj66]

Thanks for the replies.

Steve - Caliper and disc are properly aligned and its the correct disc/spacer combination (8mm spacer). However I haven't checked the whole movement of the caliper pistons - Good thinking - will have to check that later.

Princess pistons do have connections for two flexi hoses - initially tried woith two hoses but that caused problems and i reverted back to one flexi which is the divided into two at the caliper (which is how the calipers came to me)

[My thinking was that with two flexis, the master cylinder was trying to squeeze twice the fluid per cm of pedal travel than it was designed to (Ford use only one flexi).]

Wilf - someone else has mentioned this afternoon that it might be the master cylinder. Must admit i always tend to doubt my own workmanship and the Haynes manuals are some what unclear as to which way round the seals should be on the primary & secondary pistons - anyone got a ford Workshop manual with a nice clear diagram?

[Purple AK]

Hi Robin. This definately sounds like master cylinder as others have suggested. Could be either,
1) The piston not fully returning (as Wilf suggested) apart from checking for a little slack on the pushrod, Make sure the piston retaining circlip hasn't jumped out of it's groove so stopping a full return.
2) The return port in the m/cylinder is blocked.
3) You have assembled the seals incorrectly (could also cause 2)