Disc brake woes

[sydduckett]

Have had some issues with.scootrs front brake for a while wjich is a combination of grabing and spongy pull. Iv bled it a couple of times and theres no real improvment. As far as the grabing my first thought was disc warped so changed that and stillthe same, then pads and so on. Anyway iv had to strip the front end for something else which has meant i now have it all in pieces. Im looking for suggestions on how to try and identify the likely culprits for both issues. Evrything seems seated without any paint build up to offset the disc so im a bit stumped, it used to be spot on so i know somethings not working

Ta
Sef

[belly]

Spongy issue could be the hose, have you fitted a good quality braided one? As for grabbing, mine did. I changed the disc for a smaller diameter one which cured the problem, someone I know has reduced the pad area which has had the same effect

[shocky]

I use a smaller master cylinder

[sydduckett]

This is the one winter job i wasnt looking foward to. The more i think about it, the wheel needs to be in the bike. Im not really going to be able to tell anything with loads of bits. I think i will invest in new banjos and hose and see if that cures the spongyness.
When you say smaller disc do you mean it reduces the surface area of what touches the pads? Im using 5 hole 5mm thick disc with nissin sintered pads.

[sydduckett]

I know you can get engineering comps that can get the disc flat. Maybe its worth getting this done at least you can then take that out of the equation?

[GP240]

I had a binding brake and changed just about everything, it turned out to be the thin nut on the spindle that holds the caliper plate up against the speedo drive housing, someone in the past had cut a full nut in half at an ANGLE, everything was fine until I tightened the outside link/spindle nuts which was then tilting the plate/caliper/pads to the disc. Just an alternative suggestion to the norm. Oh and I tried a brand new disc from a shop in the N/East, min wear = 3mm, it measured 2.8mm.

[hendy]

Braided hoses? Pah! They're fashionable.

Go for a test ride on a kawasaki zx6r and try the brakes. You'll find them more than adequate even though they have around 4 times the length of rubber hose a lambretta has.

Often fitting them 'fixes' issues but in truth the fix was the bleeding or correctly done reassembly that was carried out.

I have sorted loads of brake systems out and find the majority are due to 2 things:

Air in the system. And here's the biggy, dirty pistons/seals.

If you have a single piston sliding caliper you also need to pay particular attention to the sliding pin and its seal.

Good luck.

[warts]

The guzzi has a very long front to back pipe as it has linked brakes. I got a Gunsons easibleed http://www.halfords.com/motoring/garage ... -eezibleed and that made bleeding it a piece of cake.
The other thing is not to use old or used brake fluid as it absorbs moisture from the atmosphere which can severely degrade its performance. Only buy enough fluid for the job in hand you need, don't store open bottles.

You can check the disc for runout buy fixing a pointer to the fork leg adjacent to the disc and spinning the wheel. Any wobble should be clearly visible.

As mentioned by Shocky, it could be the master cylinder is too big, moving too much fluid and the brake therefore too aggressive.

[diesel]

I had binding issues with a scootrs disc and it turned out to be a chip of powdercoat was jamming a piston

[shocky]

The bit onthe leaver thatpushes the piston in that pumps the fluid might need a wisker shaving off as the piston might just be pushing all the time and not relaxed (sounds rude I know) I chinned the standard monstrosity of a master cylinder of and fitted a ajp one for a runner the leaver can be adjusted so it just touches