front brakes

[pae]

Hi folks, long toime since I posted, but I'm doing some work on judodave's 650z and wonder if anyone can help.

We're having an almighty problem getting the front brakes lever to go firm, but I'm stumped to know what else to try. I seem to recall my previous 650 never had a very firm lever, but can't say if it was like thi one or better.

Anyway, the bike's got new pads, new pistons and seals in both calipers, new master cylinder seal kit, a new 'o' ring around the base of the plastic master cylinder reservoir. Just about everything, except the hoses are new.

I've tried bleeding with a syringe by sucking fluid through from the reservoir down and out the bleed nipples, and also by pumping fluid the other way (into nipple until it comes out of reervoir). I've also done the traditional undo nipple-piull lever-do up nipple-release lever etc way. None of them make it any better which makes me think I've got the best I can expect.

I can get the wheel to brake hard when pushing the bike along using about 1/2 - 3/4 lever travel and it feels firm, but the lever never goes solid. With more pressure you can always squeeze the lever harder which seems wrong, even though the brake performance might be OK.

I recall my previous one was a bit like that, but that was few years ago. Does that sound 'normal'. I've got an MOT booked in the week.

Anything else these 650's are known for in the braking department that I should check?

thanks, Phil

[andy120t]

I don't know specific 650 brake characteristics but old hoses - (rubber ones) - will contribute to any sponginess.  As they age they expand more under braking pressure and do the feel gets worse. Key thing is that the lever doesn't come all the way back to the bars.

Braided lines may not fit the look of an older bike but will help - my breaker sells 2nd hand ones for a tenner a go which helps keep the cost down. That way you'll have eliminated everything - meaning any remaining lack of performance is down to the brakes themselves!

[Bryanj]

Depends on your tester, when i did it as long as there was some "reserve" travel when the wheel locked it was OK bt me

[pae]

Thanks chaps, I'm dropping the bike off a couple of days before the test, so I'll ask him if it's worth putting it through the test then.

Phil

[K2-K6]

Some of this era i.e. earlier twin disc set-ups had a distinctly long spongy travel but they should never touch the bars though.

It's just as much to do with the master cylinder ratio compared to the piston size that dictates travel at the bar if no air is present.

If you had a master fitment for a single caliper wrongly fitted you'd get roughly twice the lever movement.

I've had some systems that we cold only get to bleed all of the air out by lifting the caliper above the mastercylinder and putting a block in place of the disc to allow a conventional bleed by pumping fluid through to take place as the air rises it is expelled more easily.

[pae]

I seem to recall my previous 650 never had a great feel on the lever. Will let you know what the MOT tester makes of it.

Phil

[fisjon]

Hot tip for bleeding brakes after refitting pistons and seals!

Half fill resevoir and GENTLY pump fluid into system topping up as you go until it is obvious no more fluid required.

IMPORTANT Do not force the lever at any time as this will compress the air into the fluid, yes brake fluid will suck in air and it will become elastic.

Leave overnight. The weight of the fluid will want to go down and the air will want to come up. In the morning the air will be at the top of the system, and by GENTLY moving the brake lever in and out WITHOUT APPLYING FORCE you will see lots of bubbles rising into the resevoir. Do this for several minutes until the air bubbles stop rising. Leave for 15 minutes and then gently move the lever again to see if there is any more air. Repeat if necessary until no more bubbles.
  
The important thing about this process is not to apply pressure on the lever until the system has no air in it or you will compress the air into the fluid and this will give you a spongy lever.

Also you should avoid leaving the cap off the resevoir for long periods as air will ingress into the fluid merely under atmospheric pressure.

This method requires a little patience but beats the s--t out of spilling fluid over your nice clean bike!

My CB650Z front brake does have braded hoses but having used the above system the lever is solid and moves very little to provide good braking.

Oh! One more thing, always use new fluid, if it has been in a can that has been opened for some time  it is usless.

[florence]

You may find it will improve given a little time, once the new pads have bedded in.  If you think about it, they will not be perfectly parallel until they have worn a little to the disk and this will contribute to brake 'feel'.  Also, as has been suggested, braided hoses make a good deal of difference, I have them on my CB500/4 and would never go back to the old kind.

I do not think you will ever completely eliminate that slight sponginess but it should only be very slight.

[judodave]

thanks for all the reply's, just to update you all the front brake sorted itself out over a period of time and have been using the bike regularly up until this recent change of weather. many thanks again.