Bleeding Front Brakes on a GT750

[pikeslayer]

Well I finally got the front calipers rebuilt, but am unable to bleed the brakes. Used a mighty vac on both sides. Nothing but fluid coming out of the bleeders. Brake lever is pretty soft and almost bottoms out.

Any tricks to this?

[Suzsmokeyallan]

Its one of those things, sometimes you can encounter an issue. You might be willing to try back filling the system if it fails to fill normally.
Sometimes the master cylinder gets a 'lock' and the pumped fluid just oscillates in the bore, also make sure the small bleed back hole is clear in the cylinder bore as this can cause an issue.
Is the master cylinder also rebuilt with a new service kit and is the bore smooth.
To backfill the system, fill a large syringe with brake fluid and with a hose attached apply it to one side of each calipers nipple at a time forcing the fluid up into the lines.
Slacken the union at the master cylinder to allow for the fliud/air to bleed out at this junction.
Once its all flowing clear with no bubbles refit the union and try again for a traditional pump lever bleed out. Are the brake hoses new or at least clear, if they are more than seven years old REPLACE them.
With the mighty vac attached the fluid should automatically drain from the master cylinder once you apply the vacuum and pump the lever.

[rngdng]

Don't feel too bad. I've always found motorcycle front brakes to be a PIA to bleed. I don't know if it's a design thing or simply because the caliper is lowest point in the system.



Lane

[tz375]

There are a million tricks that work "sometimes".

I start by cracking the top banjo bolt to let air out at the highest part of teh system. It's messy, so be careful. Fitting a banjo bolt with bleeder makes it easier.

When that stops hissing and starts to push fluid out, I lock it up and bleed the system normally.

One cute trick is to flick the brake lever, ie pull it in and let it flick back out. the sudden stop causes air bubbles in the master cylinder to break out into the reservoir.

When that stops, try bleeding again. Then pull the lever all the way in and strap it in place with a bungee cord and leave it overnight in the "ON" position.

Repeat the flicking etc if the system is till soft.

Check the hoses for bulging too and the calipers to see if they are flexing. Sometimes the problem is the mechanical parts rather than the hydraulic systems.

A common problem on the old RD Yamaha double sided calipers is them flexing. And that is usually caused by the mounting lugs being worn or distorted.

The cure is usually to loosen both mounting bolts, pull on the brakes and measure the clearance between the caliper and the fork leg brackets. If/When the gaps are different you need two different thickness of shims to get it straight again.

[Craig380]

Another trick is to get a length of clear hose that will reach from the bleed nipple up to the master cylinder reservoir.

First, cover your fuel tank in kitchen cling-film

Attach one tube to each of the bleed nipples, and put some brake fluid in each tube. Open the bleed nipples. Then submerge the other end of each tube in the fluid in the reservoir (use tape / cable ties / wire / whatever to hold the tube ends in place, under the surface of the fluid).

Pump the lever to circulate the brake fluid (keep the reservoir topped up) until the clear tubes are full of fluid.

You now have a closed system that you can leave overnight -- no more air can get into the system. Turn the handlebars to the left (so the master cylinder points down slightly) so that air in the master cylinder will rise into reservoir; air in the calipers will rise in the tubes.

In the morning, a few gentle squeezes on the brake lever will purge any remaining air, so you can nip up the bleed nipples and remove the tubes.

[pikeslayer]

Thanks for the suggestions guys. I think I'll start by checking the bleed back hole is clear. Then I'm going to try the back fill method. It never made sense to me why most bleed methods try to force air down the system when it naturally wants to go up.

[Suzsmokeyallan]

Remember to be careful when checking the bleed back hole in the master cylinder. Remove the reservoir and fill the shallow well below it with some fluid and pump it SLOWLY. The small bleed back hole which is in front of the larger entrance hole throws the fluid upwards on the return stroke of the lever and it will possibly get into your eyes, so wear protection or be careful with what youre doing.
If you did not rebuild the cylinder its possible the internals are filled with old jelled fluid which is blocking the ports in the main piston.

[pikeslayer]

Yeah, i didn't rebuild the cylinder. Probably should have. If I knew I was gonna keep it for a long time and maybe restore it, I would have. I just want to get it somewhat road worthy.

[Wayne Meuir]

One other suggestion:
When you are pumping the brake lever, pump it very, very slowly. That seems to work better and gives the air time to escape.

Wayne

[pikeslayer]

So I took the reservoir off the master cylinder. I could see fluid squiting out of one of the holes. Does this mean it is functioning properly?

Also got a syringe and tried to push fluid into the caliper, but it wouldn't go in. I might have got a little in there, but it was very difficult to push down on the syringe.

Any other ideas?

[Suzsmokeyallan]

If you didnt rebuild the master cylinder it would be wise to disassemble it and look at the condition of the parts, especially the rubbers.
Which hole was the fluid coming back out of as they are two, the main inlet hole and the smaller bleed back hole.
Since you rebuilt the calipers it has to be an issue with one or some of the hoses or the master cylinder.
Did you remove the hoses and check the union bolts and each hose for free unobstructed flow???

[pikeslayer]

Fluid was coming out of the hole closest to the banjo bolt.

Did not remove brake lines, but during the initial bleed it was fairly easy to get fluid down to the bleeders.

I'd like to avoid the master cylinder rebuild if at all possible. This thing is becoming a money pit. I just want to ride the darned thing a little to help decide its fate.

[pikeslayer]

So, is there a sure fire way to say "this is a good master cylinder", or "this is a bad master cylinder"?

[Suzsmokeyallan]

Pike there are a lot of things that can go wrong in the master cylinder, especially if you do not know how old it is and how often the fluid was changed in the past.
The bore could be pitted and that can put grooves in the rubbers, the piston in that cylinder you have has a little valve plate on it and three very tiny holes and if its clogged with old jelled fluid it wont pump properly.
Also if the rubbers are shot, as in gotten soft or hard, it wont pump much either or build sufficient pressure, so again you need to pull it apart for checking.
You cant just say its "good" or"bad" since you cant see inside it, and until you do an inspection you will never know.
Most manufacturers recommend any vehicle thats over seven years old, all the rubber components in the braking system should be replaced to give you proper performance and added safety, doing this can also help to save your life.

[tz375]

Allan is right, and I'd just add that if you can't get a firm brake feel, then it is highly likely that the M/C is "bad".

If teh bikes has to be stock, rebuild the original. If not just grab a more recent one off flea bay for say a 2000 SV650. (not the S if you want an integral reservoir).

Or improve the brakes and fit one that's a better match to the calipers and they came on GT550s. It will look stock but will work better and you get to clean it up before it goes on.