Not Porsche but need some help please.

[911hillclimber]

My daughter has an MX5 UK car, 63k miles.
It has developed a fault with the brakes, hence this question:

We had some rear caliper problems on drivers side, stripped the caliper, fixed it and bled that corner.(the adjuster had detached itself- these are weird designs.)

Drive the car about 5 to 10 miles and the brake pedal free-play goes from 15mm to zero incrementally each time you brake until the system applies the brakes progressivly and they eventually lock-on and the engine cannot over whelm the brakes. All 4 lock on.

To release the car, open the driver's rear nipple and instantly the car is drivable, no brake drag, all normal.

Could this be a master cylinder or ABS unit?

any ideas please.
Graham.

[Mick Cliff]

Sounds like the internal lining of the flexible brake line may have collapsed, lets fluid pass then forms a flap to prevent the fluid returning?

[911hillclimber]

But all four brakes come on not just one corner; can't see all four hoses failing at the same time, but maybe for all four to be 'braked' means the master is at fault.
I can get a new one for £70 so prob the best thing to do.

Thanks for the thought!

[Mick Cliff]

But all four brakes come on not just one corner....

Ah! Missed that
Is there a flex hose downstream of the MC but before the lines 'split' into front/rear lines?

[impmad2000]

Perhaps a call to Sam Goodwin in Nuneaton will help, An MX5 man we've used in the past.
Tim
(I assume he's still there)

[911hillclimber]

Thanks Tim.
There are 4 flex pipes all in the usual places.
Got this week to fix it..

[abm914]

Have you bled all the brakes, not just the rear corner that you renewed the caliper on. Could be that air is now in the other parts of the system. Also check if there is a best method to bleed MX5 brakes as there may be a specific sequence to follow.

Ad.

[Bruce M]

Check the pedal is returning properly and the return spring isn't broken.

[911hillclimber]

Excellent , thank you all.
I'm sure the pedal is coming back, and i've looked hard at the link from pedal to srvo push rod. all looks well.

I clamped the rear hose to minimise loss of fluid and when re-connected a helper pressed the brake pedal down to have pressure with the nipple open I removed the clamp so fluid bled immediatly.

however, this car does not bleed right to me. First 'down' with nipple open moved air and some fluid to the floor. Tighten and 'up'.
Do this again and there is a lot of free play pushing the pedal down and it stops quite hard at 3/4 of the down stroke with little fluid movement.
This why i think it is the master.

On my 73T (stock brakes) i had this same 'pumping-up issue. Turned out i had tried to remove all the free play as was reasonable. All was well to start with then progressivly the brakes locked on.

Adjusted the pedal free play (to 100% more) and the prob went away. I think it must be a similar condition here but i have not touched this adjustment (in fact it is not adjustable.)

[smallspeed]

Might be worth giving it a pressure bleed.. Also might be worth slamming the anchors on while drriving on a loose surface and ghetto-bleed the abs pump incase you got some air into that?

[911hillclimber]

Do you need to bleed abs cars differently? (ie gently)

[smallspeed]

No but I had problems with amies tt when I replaced a rear caliper. No matter how careful I was I got lots of air into the abs pump which could only be bled out by cycling it while bleeding it using diagnostic software. Took HOURS
I'm not saying its the same but might be worth checking incase there's some knack to doing mx5's.. The problem with the TT was the abs pump is above the reservoir! And audi in their wisdom removed the bleed nipple from the pump to save a few quid..

[911hillclimber]

Ebay servo/master arrived today. will try these and bleed first then only the ABs is left, and the bleeding technique to master.

Other forums are working on the problem too inc 2 MX5 threads.