Brakes... or not to much of !!!

[tj2]

Hi all, finally got my GT 750 A up and running yay!!
My problem is now being the front brakes.
I have managed to get them fully bled but still have problems with excess lever travel.
On the rolling road, the brakes have sufficient effort to pass the MOT but the lever comes almost to the bar.
Some flexing from both calipers is evident.
Both calipers have been rebuilt with new piston and seal kits with new pads.
I currently have a Honda vfr 750 master cylinder fitted, but had exactly the same effect from the fully rebuilt original master cyl (hence the swap)

I see that this is now a purely mechanical effect, but coming from where? and how do I cure it?

Cheers
Terry

[Zunspec4]

Hi tj2,

The choice of master brake cylinder is important. For a single disc set-up the diameter of the piston in the master cylinder needs to be in the range of 10-12mm. For a twin disc set-up the diameter needs to be in the 14-16mm range (ish). If you are running twin front discs with the small piston it will not move sufficient brake fluid and you will get the lever coming back to the bars. Similarly if you run a large diameter piston on a single disc set-up it moves too much fluid and you will have a brake lever that hardens up very quickly and hardly moves at all. I'm not sure if that helps you at all but I hope you can see where I am coming from.

Cheers Geoff

[tj2]

Yes Geoff See where you're at.
Its's a standard twin disc was with standard master cyl, lever to the bars fully bled.
Now with a late 80's vfr m/cyl and braided lines fully bled, lever back to bar.........

Terry

[Zunspec4]

Hi Terry,

Did it have a twin disc set-up from new, or could the 2nd disc be an upgrade. If originally it was single disc the previous owner probably did not up-grade the M/cylinder to match. If it is all as supplied from the factory, including the Suzuki M/cyl, I would speculate that you still have air in the lines. Does it harden up if you pump the brake lever? It would be worthwhile to place an elastic band around the brake lever so that it is pulled back to the bars and leave it that way for 24-48 hours. This allows any air in the line to travel upwards and back through the m/cyl to be expelled into the reservoir.

Cheers Geoff

[Alan H]

Don't know about the Honda M/C, but the correct one for the GT750 (or GT550 with twin discs) has a 'D' underneath (for dual disc).

[sportston]

I would speculate that you still have air in the lines. Does it harden up if you pump the brake lever? It would be worthwhile to place an elastic band around the brake lever so that it is pulled back to the bars and leave it that way for 24-48 hours. This allows any air in the line to travel upwards and back through the m/cyl to be expelled into the reservoir.

Cheers Geoff

That is very good advice. Bike brakes are a pain in the bum to bleed fully. If it hardens up when you pump the lever in rapid succession, then you may still have air in it.
There is one other possible cause, which is worth checking. A badly stretched set of old rubber brake hoses could give similar symptoms. If that is the case, replace them with braided hoses and your problem would be cured.

It is also vaguely possible a warped or badly seated disc could give similar symptoms. Each time the wheel rotates it would push the pistons in the caliper back in a little. But I would say it is the least likely cause of your current predicament, since they should spot a disc that would be that badly warped in the MOT test.

[tz375]

If the calipers are flexing, check that they can slide easily on their pins and that the pads are not binding. They were never all that good from new and compared to modern brakes, they suck. They are not a very rigid design and it you want really good brakes, calipers and disks should be replaced with something more modern.

Is it possible that the pads are worn or new and not bedded in or are old and contaminated? You could also try glass bead blasting the disks to remove embedded oil, dust etc. And if hoses are OEM, braided stainless replacements might be a good investment.

[Alan H]

This is another one that caused an issue a few weeks ago when I mentioned to someone that being original isn't always a good idea.
Upgrading the brake system by fitting braided hoses was a period upgrade and is much better for it's life saving qualities.
As I said then - albeit on a completely different matter - sod original - or anal - fit modern parts when your life may be at stake.
You really can tell the difference. I fit braided hoses (with black outers if needbe) to my old Suzuki discs.
Unless I just want to upset the purists..........

In that case, the red hoses match the metallic red in the tank/panels/nose fairing paint.

[tj2]

Thanks for the replies.....

Zunspec4 Yes was originally twin disc and no air left in the lines.... the lever does not pump up.

Alan H The original M/cyl is the correct one for twin discs, the Honda one used for comparison... same effect.

Sportston The original hoses fitted were a brand new set of genuine Suzuki, now replaced with a braided set.....same effect.

tz375 The calipers have been rebuilt with new pistons and rubbers throughout, the pistons can be pushed in by hand and pump out freely, the pins slide easily, the pads are new (not yet bedded in) and move correctly in the caliper without undue binding. The discs are in very good condition with no obvious warping etc, the lines are now braided as said above.

Alan H (or anyone else) Do you know of any more modern calipers that will fit direct to the legs of the 750? I'm not bothered with totally original and am in no way anal about it !! Just want working brakes without spending more...........and more..........


Thanks again for the replies, looks like you are all following the same lines of thought as me (or the other way round) and have pretty much gone through all that can be wrong with any remedies....

Now to piss me off more, she don't want to start... really, really hope its just timing slipped

Terry

[Alan H]

Alan H (or anyone else) Do you know of any more modern calipers that will fit direct to the legs of the 750? I'm not bothered with totally original and am in no way anal about it !! Just want working brakes without spending more...........and more..........

Well, if you look at the pic of my bike - it has GS1000 calipers (direct fit without any mods) and what then you can't see, is that they are 'powered' by a Bandit 1200 master cylinder. They work well with drilled discs. It still sounds like there is some air in the system somewhere. I usually put a syringe filled with brake fluid on the caliper bleed screw(s) and slowly pump fluid back UP into the master cylinder if I have bleeding (sic!) issues - just keep your eyes on the fluid level in the reservoir - and that will shift any tiny air bubbles. Or even just push the caliper pistons all the way in slowly which will purge the calipers of fluid and air (again watch the level in the res.)

I take it you know all the correct swear words to use while this is going on?
They don't make it any easier, but the cathartic effect is good.

[Zunspec4]

Well Terry seems to have just about all the angles covered. If the lever is going back to bars still the only thing not investigated is the state of the seals/piston within the master cylinder. Although the original and VFR m/cyl gave the same results they BOTH may need a service. Another possible factor is air still lurking in the caliper. When I bleed the AP Lockheed caliper on my Seeley I usual have to dismount it from the fork to orientate it this way and that to get trapped pockets of air out (Yes, a PITA )

Cheers Geoff

[tj2]

Well Terry seems to have just about all the angles covered. If the lever is going back to bars still the only thing not investigated is the state of the seals/piston within the master cylinder. Although the original and VFR m/cyl gave the same results they BOTH may need a service. Another possible factor is air still lurking in the caliper. When I bleed the AP Lockheed caliper on my Seeley I usual have to dismount it from the fork to orientate it this way and that to get trapped pockets of air out (Yes, a PITA )

Cheers Geoff

Sorry should have mentioned that the original m/cyl has been fully overhauled with all new parts........



Alan yes plenty of swear words and all the connotations and different orders they can be used in

[jabcb]

Does the VFR MC have an adjuster on the brake lever? The Nissin MC we bought for the GT250 cafe & my modern Triumph have one — it adjusts the amount of lever travel you get when applying the brakes. If you have one, it may be adjusted to give the maximum lever travel.

[tj2]

Does the VFR MC have an adjuster on the brake lever? The Nissin MC we bought for the GT250 cafe & my modern Triumph have one — it adjusts the amount of lever travel you get when applying the brakes. If you have one, it may be adjusted to give the maximum lever travel.

Either way it reaches the bars..

Cheers

[Zunspec4]

This is becoming a super puzzle

Starting from first principles: If the lever is going back to the bar it can only be caused by two things.

1. The caliper pistons are retracting to far and thus need more fluid to bring them back to the pad. Except pumping the lever does not firm up the action.

2. If not 1. above there still has to be air in the system (somewhere).

Did you ever have normal lever response before refurbishing everything ?

Cheers Geoff