Spongy brake on GPZ500...

[StormCrow]

I've been having a continuing problem with the front brake on my '89 GPZ500. (Rear drum 16" wheels model).

The brake has always seemed a little inefficient (seems to be a fairly common trait of the model) but it's been a nightmare recently.

Have done the seals, have put new piston in (the other is awaiting replacement but not too bad... cock up with the item I bought - 0.3mm oversized), new EBC sintered pads, given everything a good clean and grease etc but it's still shit.

The disc looks ok - it's a little unevenly worn due to the previous piston sticking but I would say it's alright...

The only weird thing is that the master cylinder has been changed to one from a GPZ550 (previous owner) - not sure why - would this be having an effect? The GPZ550 seems to be a twin disc model...

I've bled the brakes a number of times, no bubbles at all (done it all before loads of times on different vehicles) but still the braking force is shite and the handle moves 75% of the way to the bar, only 'gripping' at 50%

Master cylinder problem? Any other suggestions guys?

TIA

[Kickstart]

Hi

Suspect if anything the GPZ550 master cylinder would shift more fluid and result in less lever travel.

The last GPZ500 we had also had persistently spongy brakes (although still perfectly usable). Replaced the hoses and rebuilt he caliper which made next to no difference.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[StormCrow]

Thanks Keith

I figured that about the M/C too...

The problem is that the brakes aren't usable really - they're so soft as to basically be useless! I don't need it to stand on it's nose but to be able to stop fast, no chance at present.

Just popped the pads out to have a good look - they're 'hotspotted' in one place (can photo if required) - the 'larger' of the two pistons is the one doing this...

Guess I'd better see if the smaller piston is moving freely as well (again!)

[Kickstart]

Hi

Takes a lot for a piston to stick off the pad (as opposed to sticking out once pulled. I would check for the pistons sticking out.

The larger rear piston is to even out the effect of the pad over its width (or should that be length). The leading edge provides a fair proportion of the braking.

Do both pistons have the stuffers in place, and are they in good condition?

All the best

Keith
____________________
Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[StormCrow]

Yep everything is as it should be.

It *may* be user error - I took it apart again earlier and found that one side of the anti rattle shim was bent slightly, think that the shim had been keeping the pad away from the disc contact area.

Sanded down both pads on a flat surface to 'reset' them and put it back togther, having pumped the pistons out and reset them (not completely out) and it seems to be harder (ooh matron!).

Have left the lever cable tied back tonight to expel any remaining air and will see how it feels tomorrow...

Cheers

[StormCrow]

Things seem *much* better today - haven't been out for a ride yet but the maximum travel of the lever is now about 30%.

Fingers crossed!

[Casper]

Did you try bleeding from the bottom up? all you need is a big syringe. 100ml will do. Takes less than half the time and gives better results.

[StormCrow]

Yeah, I do need to get a big syringe the next time I'm in the vets with the dog.

Took it to work today - all was fine with the brake - the new rear tyre made things interesting, especially in the wet on the way home!

[andym]

Might sound silly here, but when you are pressing the back brake make sure that the caliper itself isn't moving as I just sorted this problem with my 750. No matter how much I bled the system (replaced seals, popped pistons etc), the brake always felt spongy. It turned out it was the caliper slider? that was slightly buckled allowing the whole caliper to 'flex'. I've replaced one of the sliders and it has sorted my problem... Just a thought

[StormCrow]

Hi Andy

Wouldn't be silly, except I've got a drum rear...

The front seems fine now - just need to bed it all in properly I think.