inconsistent brakes

[sprint4dm]

Hi, just a bit of advise please.

I have an Elan Sprint with an inconsistent brake pedal feel, sometimes it feels fine occasionally the pedal feels lower and a second press brings a better pressure.

There are no leaks, master cylinder looks good no signs of fluid in the footwell, I have bled the brakes several times but no improvement. All the flexible pipes are stainless braided and the servo looks OK though I have not attempted to take it apart.

I have got to the stage where I feel it must be a servo problem, any thoughts would be appreciated

Cheers

Dave

[jimj]

Your brake discs are probably slightly out of true and pushing the pistons back in the brake caliper(s).
Jim

[elansprint71]

My venereal friend James has arrived ahead of me. I too suspect that your discs/rotors are running slightly out of true (they are, after all, only mounted on three points). The way to confirm this is with a dial test indicator; rotate the disc and see what run-out you record. The tolerances are in the workshop manual.
The proper engineering solution involves a small hammer and a considerable amount of patience.

[john.p.clegg]

"The proper engineering solution involves a small hammer and a considerable amount of patience"..... or shims and a lot of patience...

John

[sprint4dm]

Your brake discs are probably slightly out of true and pushing the pistons back in the brake caliper(s).
Jim

Thanks to all for the responses

I will check the discs, but they were all replaced less than 5,000 miles ago by the previous owner and still look like new.

Dave

[mark030358]

Interesting topic brake discs. I have just bought some EBC grooved disk (front and rear). They state on the leaflet that comes with the discs that if they are out of true (more than 2 thou!!!) then there warranty is invalid unless you have them skimmed on the car. See the utube links on the website below.....

www.skimmydiscs.co.uk

cheers

Mark

[john.p.clegg]

Looks good but can they cope with Knock-ons...

John

[alexblack13]

Your brake discs are probably slightly out of true and pushing the pistons back in the brake caliper(s).
Jim

Thanks to all for the responses

I will check the discs, but they were all replaced less than 5,000 miles ago by the previous owner and still look like new.

Dave

The discs are never the problem. The disc mounting points are the problem normally. Esp' if a PO has been whacking the living daylights out of them trying to 'punch out' the fixing bolts.

The fix is to carefully machine the mounting surfaces into true. Have the shafts mounted in a good lathe and then clocked in using the bearing 'land' as the reference. This must be done to +/- F. A. IE 0.0....... Th better this is done the better the result.. Then machine the mounting points very gently and carefully until they are all faced up.. Refit and enjoy. and don't hammer out fasteners or you will knock them out of true again.

Have fun....

[alexblack13]

You might also have a master cyl issue.....

Forgot that bit.

[oldelanman]

Also worth checking front wheel bearing adjustment - excessive play can also cause pad knock-back and consequent long pedal travel.

[AHM]

You might also have a master cyl issue.....

Forgot that bit.

I'm with Alex!

Disc runout would only cause inconsistency if the disc had done less than one revolution - The pistons would have been pushed back after one revolution.

If however something in your hydraulic circuit was a bit lazy, it wouldn't have fully returned before your next pedal application.

[robertverhey]

Might be worth isolating the booster, by by-passing it and see if you get a consistent (albeit less effective) pedal

[collins_dan]

I had a situation that sounds similar. I rebuilt the master cylinder, then had similar pedal symptoms. Soft at first, then harder. Ended up I put the seal in backwards at the end of the push rod, so wasn't getting a complete seal. Could be your seal is perishing. Good Luck, Dan

[Andy8421]

I had exactly this problem, caused by run-out in a rear disc pushing the pads back (as described above). In my case the disc and shaft were true but the outer bearing was clapped out allowing the disc (and wheel) to wander about. This was hidden by the rotoflex joints putting the bearing under sideways load when the car was jacked up masking the play in the bearing. Taking the load off the rotoflex by supporting the rear hub at normal running angle allowed the problem to be located.

[RotoFlexible]

To add to the litany, it's also possible that the piston seals in one or more calipers are retracting the pistons slightly after each application. It feels like air in the lines because the brakes can be "pumped up". The solution is to lubricate the pistons with appropriate caliper grease during assembly.