Help! I've been trying to bleed my GT750 brakes for 4 weeks

[n8balls]

Hi guys

I would have expected to have my bike registered and on the road by now but I have struck a problem – I just can’t get my brake lever firm...

When I got the bike (GT750A), the old master cylinder was seized so I bought a new master cylinder for a GS750. After bleeding, the lever was quite spongy and I need to get it firm in order to pass the road worthiness inspection for registration.

In the past 4 weeks I have tried the following:

• Replaced all brake lines because I suspected they may have lost their rigidity
• Disassembled, cleaned and rebuilt the callipers with new seals (just in case there was something spongy lurking in there, there wasn’t ).
• Tried reverse bleeding – pushing fluid with syringe from the callipers up
• Vacuum bleeding with vac bleeder kit
• Bleeding at all points - the M/C, then top of the Y piece, then bottom of the Y piece, then callipers and then the bleed points
• Zip tying up over night
• Tapping the lines, t-piece, callipers with a rubber mallet to persuade stubborn bubbles to rise
• I also put stock bars back on - it had drag bars which meant the top brake line had a high point before the M/C that could trap air


The only other thing I can think of is if air is trapped in the high point of the lower brake lines...

highpoint.JPG

Or could a stubborn bubble be trapped in the Y piece?

Anyway, any advice would be greatly appreciated because the bike is otherwise ready to go and I can’t wait to get it on the road

Cheers, N8

[Suzsmokeyallan]

I'd suggest checking that master cylinders bore and its internal seals, its apparent from what you said that you have covered all other possible areas.
What type of fluid are you using.

[titan performance]

What brake lever are you using?

I had a similar issue once, and in the end I put a small self tapper into the lever where it meets the piston, effectively giving the piston more travel. That fixed it !

[n8balls]

thanks for the replies The master cylinder is new from ebay, and the lever came with the master cylinder and seems to be pushing the piston in to the end of it's travel (I might try and shim it tho just to make sure)

The m/c is for "GS1000 GS1100 GS1150 GS750" according to the ad
http://www.ebay.com/itm/FRONT-BRAKE-MAS ... 19c9d0a266

master.jpg

master2.jpg

Is it possible the M/C isn't suited to the GT calipers? Like maybe too low displacement? Maybe the M/C is really for a single caliper bike - there's only so much trust you can place in an ebay description
As for the fluid, I'm using dot 4, castrol and now valvoline. I'm getting through a lot of it with all these bleedings
cheers, n8

[Coyote]

In a nut shell, the MC is too small for the volume of fluid you're trying to displace. The bore on the GT master is 15.88 diameter. That's nearly 2mm larger than what you have mounted.
My GS1000 (78) is a single caliper bike and the factory service manual I have says the bore is 14mm - replace at 14.05. So I think you hit the nail on the head when you said "maybe it's for a single caliper bike".
I advise you to look for a GT750 master cylinder - even if you have to rebuild it The MC's on the 380 and 550 are too small as they were single caliper bikes. Standard bore on them 1s 14.05.

[tz375]

The right bore M/C is crucial but there are other variables to add to the equation.

Air gets trapped at every change in section, Y pieces, calipers and of course the banjo at the M/C all need to be bled.

The last possibility is a misaligned caliper. In theory it shouldn't be an issue, but check the pads to see if they are square to the disk. If they are even slightly angled, the caliper or the disk are twisting/distorting. It can be caused by a warped caliper, sticky sliding mechanism or wear on the inner bosses where the caliper bolts to the forks.

I have that issue on one of my project bikes where the welded custom mount is not square. The easy test is to loosen the caliper mounting bolts so the caliper is free to move and then apply the brakes. If they suddenly become firm, that's the problem.

[djfisher22]

Here is my 2 cents. Motorcycle brakes can be a bitch to bleed correctly. Too many places for air to be trapped as was stated. A buddy of mine turned me on to an elegant solution that has never failed me. I use a liqui-vac meant for changing tractor oil. You break free the bleeder screw, snug it and put the vac hose on the bleeder. Pump it up, crack the bleeder and it starts sucking the brake fluid and any air along with it. You leave the lid off the master and make certain it stays full of fresh fluid till the bubbles stop in the clear plastic hose. Sometimes you may have to sqeeze the brake handle,play it by ear. I've done single and dual caliper systems with it and it hasn't failed me yet. Here's the bigger one at Tractor supply. Mine is smaller than this one and I love it.

http://www.tractorsupply.com/vehicle-ma ... ty-3000504" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;

mine................




Here's a good deal on the little one.........

http://www.amazon.com/Airpower-America- ... 603&sr=8-2" onclick="window.open(this.href);return false;

[n8balls]

thanks again for the replies

Tested the calipers for alignment and they appear to be spot on

I did try vacuum bleeding but the resultrs were much the same as with conventional bleeding. If there was some air trapped (at the top of the y piece I reckon), it wasn't going anywhere.

I managed to get the lever as firm as I've ever had it by cycling the caliper pistons - removing the calipers from the disks, pushing the pistons into the calipers, then pumping the lever until they are almost out, pushing them all the way in, and so on - all with the m/c cap on loosely. This seems to flush out some of the trapped air. It still won't lock up the wheel when I'm riding however, which I'm guessing is because the displaced fluid is too low due to the smaller m/c bore.

I have bought the special circlip pliers and a m/c rebuild kit so I'll have a go at ressurecting the original m/c when they arrive

cheers, n8

[Suzsmokeyallan]

Richard as an addition to this in a related way, the Bandits and V-Stroms were using a 5/8 master cylinder up until around the mid 2000s and suddenly they changed it to a 14mm size with no visual differences in braking performance.
I'm sure the pressure/volume ratios play an important part but I'm curious as to how its working on these later bikes.
The master cylinders physical components remained the same, so the only thing they did was to reduce the bore size.
Any thoughts on this?

[Scorch]

This is a pretty easy one, but make certain your handlebars are turned so that the reservoir is higher than the outlet and banjo bolt. I bled my Madura with and without a vacuum pump for days before it dawned on me to turn the bars all the way to one side and get the reservoir is higher than the outlet and banjo bolt. After that they firmed up in a minute.

[tz375]

Allan,

Did they change piston size in the calipers or lever ratio/pivot position at the master? I use a calculation from Kevin Cameron that I adapted and it shows 28 pounds of hand pressure to achieve a 1G stop with 5/8 and 22 pounds for 14mm with no other changes. 28-32 is just about right on the street. 22 is more like an RS125 or very light powerful track bike. With such a light pressure that would make the brakes feel more powerful because you achieve the same retardation with less effort, but make it more likely to lock up the front end.

Stock GT750 is closer to 50 pounds, or a whole lot more effort to get the thing slowed down.

[Suzsmokeyallan]

Nope Richard they changed nothing on the master casting, just the bore size. Some bikes got Tokico six piston fronts while others got Nissin four piston fronts so there was a variation on the calipers as well but its not related to one particular master cylinder for the bore size..
I'm trying my best to figure this out simply because they did this on the Bandits and V-Stroms around 2007 and it defies a logical leverage formula.

[tz375]

Allan, are you talking the remote reservoir or integrated? I have two sets of SV calipers with 30mm pistons in both positions and one remote reservoir master and it's 5/8"