front brake reservoir - does size matter?

[smokerhill]

Hi all,

Im at the last stages of my GS750l rebuild and ive put new brake lines and a front brake reservoir (single unit with brake lever etc). The problem i have is that i cant get the lever to pull hard. Its biting but not as it should. I have bleed through about 500mls of fluid, no visible bubbles but still feels spongy.

Im thinking that perhaps the new unit is for smaller bikes that dont have twin disks? e.g the lever isnt forcing enough fluid down the line. Im thinking this because it does bite, just not very well like there is either air or not enough force in lines.

Am i crazy here and any unit would be suitable to bite two discs? If so could it be the brake lines? Im 99% sure its not air...the old unit worked perfect, but isnt around for me to try.

thanks in advance for any ideas.

SW

[Brendan W]

No you're not crazy. There are differences in master piston diameters.
Plugging one of the lines might answer a lot.
Are the calipers original ?
Do you know what the replacement master came from and is it new or used?

[Guest]

Im thinking that perhaps the new unit is for smaller bikes that dont have twin disks? e.g the lever isnt forcing enough fluid down the line. Im thinking this because it does bite, just not very well like there is either air or not enough force in lines.

Am i crazy here and any unit would be suitable to bite two discs? If so could it be the brake lines?

You aren't crazy, the ratio of the master cylinder piston diameter to the caliper piston diameter must be correct to get the force and feel right at the lever. There should be a number cast into the body of the master cylinder. It is usually 5/8 or 14. There are lots of others around as well, but those are the most common sizes in M/C's that would be used on our bikes. The 5/8" bore is suitable for dual discs and the 14mm bore is meant for single disc systems.

Have you ridden the bike with this set up? How do the brakes feel while rolling? Lots of bite but a mushy lever? Not much bite and a mushy lever? Does the lever come back to the bar or not? Does the lever force seem light? How is the feel?

Are your brake lines still the OEM rubber ones? They could be contributing to the problem, but I doubt they are the main reason for it.


Mark

[smokerhill]

Thanks Brendan - the callipers are original. The master was a eBay special.... I think I know what you will say next!

[smokerhill]

You aren't crazy, the ratio of the master cylinder piston diameter to the caliper piston diameter must be correct to get the force and feel right at the lever. There should be a number cast into the body of the master cylinder. It is usually 5/8 or 14. There are lots of others around as well, but those are the most common sizes in M/C's that would be used on our bikes. The 5/8" bore is suitable for dual discs and the 14mm bore is meant for single disc systems.

Have you ridden the bike with this set up? How do the brakes feel while rolling? Lots of bite but a mushy lever? Not much bite and a mushy lever? Does the lever come back to the bar or not? Does the lever force seem light? How is the feel?

Are your brake lines still the OEM rubber ones? They could be contributing to the problem, but I doubt they are the main reason for it.


Mark

awesome info!! Thanks. I wasn't aware of these numbers. I haven't rode it like it is as I didn't have the chain on (until last night). As you push the bike it does bite but I don't think it would stop it with the engine force behind it. It stops just on the handle bar as you squeeze and does return perfect. It doesn't feel mushy and consistent but does get stiffer as the lever pulls in. Best way to discribe it would be if it was 25% stiffer it would be perfect so pretty much what you normally feel at each stage of pulling it just a little too light.

i did change the brake hoses for non OEM (I know...hine sight!) but they seem to be solid and pretty basic to get wrong?

Think i I might see if I can get a second hand master and start there...

[Guest]

IIRC the number 5/8 or 14 refers to the bore size of the piston in the master.

[bobgroger]

Use some sort of clamp to hold the brake lever in overnight. This can let air bubbles work their way up. Pump it up a few times and repeat. The original MC was 5/8. I have a Chinese MC on one bike and it works well. However I would prefer any manufacturer's OEM part. Another thing from experience, check the fork seals. If they leak the fluid ends up dead center on the pads...