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Joel845
04-08-2018, 8:04 PM
Howdy friends. I have a bike that's cobled together with lots of different parts. The problem I'm having is with my rear brake, no matter how many times I bleed it I just can't get the pressure strong enough to really safely stop the bike. Can't get them to lock no matter how hard I push the pedal. I can stop but I have to give myself lots of space so not safe for riding in town.

The master is off of a 97 Sportster and the caliper is off of a 2003 GSXR 1000. The caliper was rebuilt and has been pads, the matter was cleaned but not rebuilt. Have a nice steal braided line connecting them.

Any ideas why I'm not getting more stopping power?

kmanator
04-08-2018, 8:26 PM
I would suspect there is still air in the system or the Master cylinder bore diameter is not proper for the Gsxr caliper.I had a LTD 454 that would not bleed and had to use a vacuum bleeder to finally purge the air out of the caliper to get it to work.

Joel845
04-08-2018, 8:32 PM
I went out and bought a vacuum bleeder and that didn't fix the issue. I was told that the master will work the the caliper but I don't know that for a fact. They both have the same banjo bolt size.

kmanator
04-08-2018, 8:41 PM
Are you running the rear caliper from the gsx or a front?

TriNortchopz
04-08-2018, 8:43 PM
Howdy friends. I have a bike that's cobled together with lots of different parts. The problem I'm having is with my rear brake, no matter how many times I bleed it I just can't get the pressure strong enough to really safely stop the bike. Can't get them to lock no matter how hard I push the pedal. I can stop but I have to give myself lots of space so not safe for riding in town.
The master is off of a 97 Sportster and the caliper is off of a 2003 GSXR 1000. The caliper was rebuilt and has been pads, the matter was cleaned but not rebuilt. Have a nice steal braided line connecting them.

Any ideas why I'm not getting more stopping power?

This may be helpful; sounds like the ratio between the size of the MC and caliper may not be right - do you know the diameter of each?

Matching a Front Master Cylinder to your Caliper(s)
Hydraulic advantage -
the first thing you need to understand is how brakes work at all. When you squeeze the lever on the handlebar, you are moving a small piston which fits into a finely machined bore, and forces fluid down the line. That's close enough to true to do for now. This fluid moves down the line, and because it is not compressible, it forces the pistons in the calipers to move out, pushing the pads against the discs, generating friction, slowing you down.

Consider the fact that the bore of the master cylinder is much smaller than the combined bores of all the pistons in your brake calipers. This means that (made up numbers coming up) for every 15mm you push the master cylinder in, you move maybe 5cc of fluid. But as that 5cc of fluid hits the pistons at the bottom, they don't move 15mm but in total like 0.5mm. This is where you get your hydraulic advantage from, and this is why you don't have to squeeze the brake lever hard enough as if you were actually grabbing the pads with your hand.

Changing hydraulic ratios -
Now that you know that, you can consider how changing the sizes of any of the pistons in the system will affect this relationship. Let's assume we've taken a standard braking system which works fine and for whatever reason, we are thinking of making the following changes.

Bigger master cylinder
If you increase the size of the master cylinder, you will actually be REDUCING the power of the brakes. Yes, with a bigger master cylinder, you will have to squeeze HARDER to get the same braking force at the calipers. This is because you are reducing the hydraulic advantage you have over the calipers. Crazy, right? Maybe but it's still a fact...

...Smaller master cylinder
If you decrease the master cylinder size, you actually GAIN in braking power due to the increased hydraulic advantage you gain. Brilliant, why don't we all do it?

Well, because you also increase lever travel. In some cases it's so bad that the lever hits the bar, maybe trapping your fingers before you stop hard. This is a Bad Thing! In some case, where the brakes as standard are "wooden", fitting a smaller master cylinder can improve them massively (xj600s) but if the brakes are already fine, you may end up with them feeling vague or spongy, or trapping your fingers.

You can sub bigger master for smaller calipers, or vice versa, and the principles will hold.

But because no one really likes doing math, I have made a spreadsheet that does this for me. Download Spreadsheet here. Here's a screenshot, to show what you're getting. This shows various bikes whose brake systems I have messed about with, with those combinations I liked highlighted in green, and those I didn't highlighted in red.

See chart and more here: http://www.customfighters.com/forums/showthread.php?t=56704

Joel845
04-08-2018, 10:08 PM
Are you running the rear caliper from the gsx or a front?

I'm running the rear caliper.

Joel845
04-08-2018, 10:10 PM
This may be helpful; sounds like the ratio between the size of the MC and caliper may not be right - do you know the diameter of each?

Matching a Front Master Cylinder to your Caliper(s)
Hydraulic advantage -
the first thing you need to understand is how brakes work at all. When you squeeze the lever on the handlebar, you are moving a small piston which fits into a finely machined bore, and forces fluid down the line. That's close enough to true to do for now. This fluid moves down the line, and because it is not compressible, it forces the pistons in the calipers to move out, pushing the pads against the discs, generating friction, slowing you down.

Consider the fact that the bore of the master cylinder is much smaller than the combined bores of all the pistons in your brake calipers. This means that (made up numbers coming up) for every 15mm you push the master cylinder in, you move maybe 5cc of fluid. But as that 5cc of fluid hits the pistons at the bottom, they don't move 15mm but in total like 0.5mm. This is where you get your hydraulic advantage from, and this is why you don't have to squeeze the brake lever hard enough as if you were actually grabbing the pads with your hand.

Changing hydraulic ratios -
Now that you know that, you can consider how changing the sizes of any of the pistons in the system will affect this relationship. Let's assume we've taken a standard braking system which works fine and for whatever reason, we are thinking of making the following changes.

Bigger master cylinder
If you increase the size of the master cylinder, you will actually be REDUCING the power of the brakes. Yes, with a bigger master cylinder, you will have to squeeze HARDER to get the same braking force at the calipers. This is because you are reducing the hydraulic advantage you have over the calipers. Crazy, right? Maybe but it's still a fact...

...Smaller master cylinder
If you decrease the master cylinder size, you actually GAIN in braking power due to the increased hydraulic advantage you gain. Brilliant, why don't we all do it?

Well, because you also increase lever travel. In some cases it's so bad that the lever hits the bar, maybe trapping your fingers before you stop hard. This is a Bad Thing! In some case, where the brakes as standard are "wooden", fitting a smaller master cylinder can improve them massively (xj600s) but if the brakes are already fine, you may end up with them feeling vague or spongy, or trapping your fingers.

You can sub bigger master for smaller calipers, or vice versa, and the principles will hold.

But because no one really likes doing math, I have made a spreadsheet that does this for me. Download Spreadsheet here. Here's a screenshot, to show what you're getting. This shows various bikes whose brake systems I have messed about with, with those combinations I liked highlighted in green, and those I didn't highlighted in red.

See chart and more here: http://www.customfighters.com/forums/showthread.php?t=56704

You know what I don't know I'll have to do some looking and see. I have very little resistance when I push my brake pedal so maybe the master is too small? I donno I'll start looking thanks for the info

Joel845
04-08-2018, 11:30 PM
So I looked and it looks like the gsxr rear caliper is 10mm x 1.25, I couldn't find what the gsxr matter size for sure but it seems like it also uses the 10mm banjo. I did find out the stock sportster caliper's banjo bolts (OEM 41736-04) is 10mm X 1.25mm and the master cylinder's banjo bolts (OEM 41737-04) is 12mm X 1.25mm. So master should with the caliper but perhaps the caliper needs a smaller master?

TriNortchopz
04-09-2018, 12:46 AM
Guessin' the bore of the master cylinder may not be a good match for the diameter of the piston in the caliper; like kmanator said above: "I would suspect...the Master cylinder bore diameter is not proper for the Gsxr caliper."

Nothin' to do with the size of the banjo bolts.

You also said, "The caliper was rebuilt and has been pads, the matter was cleaned but not rebuilt."

Process of elimination, starting with the easiest - check to see if the brake pads are glazed or not. If so, clean the disc with Brake Kleen and a scrub pad, take the pads and run them over a piece sand paper to break the glaze.
Was the caliper rebuilt properly using the correct rebuild kit? Got the right fluid? That Sporty M/C may call for DOT5, while the GSXR caliper may call for a DOT4 or 5.1...research that too.

A rebuild kit for the rear master cylinder should be only about $30...so a rebuild there would be one more thing to eliminate a possible problem, since it was only "cleaned".
Several listed here for XLs, with bore sizes of .500", .550"(=14mm) and 3/4"(depending on year - are ya sure it's a '97?); also 5/8" available for custom applications:
https://www.bikerpartsuperstore.com/brakes-rotors-calipers/master-cylinders/master-cylinder-rebuild-kit?zenid=i6o5s47j43tks0cbh2tqsn58c4
Common M/C bore size for GSXR is 14mm.

What is the actual diameter of the caliper piston? Are there more than one?

Here is a HD forum thread with the same ratio info:

Thread: Master Cylinder Sizes for Dummies

Master Cylinder to Wheel Cylinder Ratio

The overall ratio determines brake "feel". There is a "sweet spot" in the range of ratios. A ratios in the 27:1 range gives 2 finger power brakes, and feeling some line and/or caliper flex. A 23:1 ratio is at the other end of the spectrum; firm. Ratios lower than 20:1 can result in a feel extremely stiff and give a toggle switch effect; nothing seems to happen until the wheel locks. (By the way, this ain't good!)

Of course for most of us dummies a quick and dirty math formula is needed. After all, how many of us can figure out these ratios? Heck, I don't even know what the basic formula is, let alone what figures to plug in! So, on to Part II.

II

Bore Sizes

2 pot single caliper: 9/16 bore.
2 pot dual calipers: 9/16 or 5/8 bore.
4 pot single caliper: 9/16 or 5/8 bore.
4 pot dual caliper: 11/16 or 3/4 bore.
6 pot single caliper: 9/16 or 5/8 bore
6 pot dual caliper: 11/16 or 3/4 bore.
*"Pot" is a slang term for "piston".

A smaller bore master cylinder will increase line pressure and therefore braking power, but it also may not move enough fluid to completely activate the piston(s) in your caliper(s).
The feel of the brake system is greatly affected by bore size.
A too small master cylinder will give a stiff feel; too large a bore, a mushy feel.

Read more here:
https://www.hdforums.com/forum/frame-suspension-front-end-brakes/1037-master-cylinder-sizes-for-dummies.html

kmanator
04-09-2018, 4:39 AM
I'm running the rear caliper.

Is the bleeder at the top side of the caliper in the mounted position? It will not bleed unless the air can escape from the top.

Bobbuilds
04-09-2018, 6:23 AM
Is the bleeder at the top side of the caliper in the mounted position? It will not bleed unless the air can escape from the top.


Will this affect reverse bleeding also? Iím having a similar problem trying to use the reverse bleed method with the caliper in place? My bleeder screw is on the bottom of the caliper.

Sorry to derail, Iíve been reading along

TriNortchopz
04-09-2018, 7:13 AM
Is the bleeder at the top side of the caliper in the mounted position? It will not bleed unless the air can escape from the top.

Might just need to unbolt it so the bleed screw is at the top.

Here is a bit of interesting info:

Here's a tip from a number of years ago. Some of the brembo calipers on various brands had a crappy bleeder nipple that didn't seat well. Note that they didn't leak fluid but will admit air. So for a couple bucks stick a new bleeder screw in it and see if that does the trick.
From: http://www.speedzilla.com/forums/ducati-superbikes/58686-bleeding-rear-brake.html

Joel845
04-09-2018, 11:55 AM
Is the bleeder at the top side of the caliper in the mounted position? It will not bleed unless the air can escape from the top.

It has two bleeders and they are both on top when mounted so I don't think that's the issue.

Joel845
04-09-2018, 12:09 PM
Guessin' the bore of the master cylinder may not be a good match for the diameter of the piston in the caliper; like kmanator said above: "I would suspect...the Master cylinder bore diameter is not proper for the Gsxr caliper."

Nothin' to do with the size of the banjo bolts.

You also said, "The caliper was rebuilt and has been pads, the matter was cleaned but not rebuilt."

Process of elimination, starting with the easiest - check to see if the brake pads are glazed or not. If so, clean the disc with Brake Kleen and a scrub pad, take the pads and run them over a piece sand paper to break the glaze.
Was the caliper rebuilt properly using the correct rebuild kit? Got the right fluid? That Sporty M/C may call for DOT5, while the GSXR caliper may call for a DOT4 or 5.1...research that too.

A rebuild kit for the rear master cylinder should be only about $30...so a rebuild there would be one more thing to eliminate a possible problem, since it was only "cleaned".
Several listed here for XLs, with bore sizes of .500", .550"(=14mm) and 3/4"(depending on year - are ya sure it's a '97?); also 5/8" available for custom applications:
https://www.bikerpartsuperstore.com/brakes-rotors-calipers/master-cylinders/master-cylinder-rebuild-kit?zenid=i6o5s47j43tks0cbh2tqsn58c4
Common M/C bore size for GSXR is 14mm.

What is the actual diameter of the caliper piston? Are there more than one?

Here is a HD forum thread with the same ratio info:

Thread: Master Cylinder Sizes for Dummies

Master Cylinder to Wheel Cylinder Ratio

The overall ratio determines brake "feel". There is a "sweet spot" in the range of ratios. A ratios in the 27:1 range gives 2 finger power brakes, and feeling some line and/or caliper flex. A 23:1 ratio is at the other end of the spectrum; firm. Ratios lower than 20:1 can result in a feel extremely stiff and give a toggle switch effect; nothing seems to happen until the wheel locks. (By the way, this ain't good!)

Of course for most of us dummies a quick and dirty math formula is needed. After all, how many of us can figure out these ratios? Heck, I don't even know what the basic formula is, let alone what figures to plug in! So, on to Part II.

II

Bore Sizes

2 pot single caliper: 9/16 bore.
2 pot dual calipers: 9/16 or 5/8 bore.
4 pot single caliper: 9/16 or 5/8 bore.
4 pot dual caliper: 11/16 or 3/4 bore.
6 pot single caliper: 9/16 or 5/8 bore
6 pot dual caliper: 11/16 or 3/4 bore.
*"Pot" is a slang term for "piston".

A smaller bore master cylinder will increase line pressure and therefore braking power, but it also may not move enough fluid to completely activate the piston(s) in your caliper(s).
The feel of the brake system is greatly affected by bore size.
A too small master cylinder will give a stiff feel; too large a bore, a mushy feel.

Read more here:
https://www.hdforums.com/forum/frame-suspension-front-end-brakes/1037-master-cylinder-sizes-for-dummies.html

Thanks so much for all the info and advise. The pads are good to go, my guess at this point is the bore of master is too small for the caliper and it's pistons since it's made for the crappy Harley caliper. I have the matching gsxr master but my buddy was going to use on my bike but he ran out of time to fab up the high mids he was going to make so he just put the sportster one on because it was easier.

Guess I need to get that gsxr master back and try to mount it on my sportster.

DustyDave
04-09-2018, 12:15 PM
Will this affect reverse bleeding also? I’m having a similar problem trying to use the reverse bleed method with the caliper in place? My bleeder screw is on the bottom of the caliper.

Sorry to derail, I’ve been reading along

If the bleeder is on the bottom it wont let the air that has risen to the top out! Make a spacer to hold the pads and cylinders in place some sockets work well for this, Hold the caliper with the bleeder at the very top and bleed. Let it set awhile before removing the spacers and reassembling. I've had several bikes over the years with the calipers moved to the bottom for lighter handling or the lower legs swapped to move the axle and change trail that required this bleeding method.
Dusty

humancertainty
04-09-2018, 1:03 PM
Did you bench bleed the master cylinder?

Joel845
04-09-2018, 5:58 PM
Did you bench bleed the master cylinder?

No I have only bleed it in place on the bike.

Tomorrow I'll have the gsxr master so I'll figure out a way to make that work with the stock sportster pedal and mounting and hopefully I'll be able to stop my bike.

ChopperDani
05-07-2018, 7:07 AM
3 weeks and no answer.

well i came here searching for help, i just recently hardtailed my sporty and the brake mount is now pointed at the bottom, also had to run a new brake line. after no luck getting any pressure, i came looking for help.

what i ended up doing and got it to work finally, after 2 1/2 bottles of fluid lol

so i used a plastic injector- like for a turkey or some shit......filled it with fluid and pushed it through the caliper......this did not work after a few tries, i believe there is a check valve, wouldnt take any fluid.

then i ran a hose from the bleeder into a cup on the floor filled with fluid, and sat there pumping and pumping...........
still no luck.

so, i was able to unmount the caliper and set it high, above the master cylinder and the bleeder now at the top. i let it sit for a day, hoping the air would make its way to the top.

ran a hose to a bottle, and pumped the brake and held it, open the bleeder and closed it. repeated this about 50 times. and all that was really coming out was very tiny champainge bubbles. but it finally built up pressure and my pads moved, nice. so reinstalled, and i have brakes! :)

Bobbuilds
05-07-2018, 7:07 PM
Nice job.

I rebuilt the rear master cylinder this weekend and finally got some stopping power myself. An awesome story I’d like to add to this is about pressure switch.

I’d lost faith in my simple ability to bleed a brake last weekend. I knew my rear master was leaking. Tore it down and rebuilt it. Reverse bleed the system and everything felt strong. Bolted the caliper back up, brought the brake pads in. Good pressure.

No brake light? Aarrgghh! No way, how did I fuck this up? Re bleed just to be sure. Ready to pull my hair out. Jump the connections at the pressure switch. No light!

Pop the lense cover to check the wiring at the tail light. Solder connections broken. What are the odds?

Brakes eh?

ChopperDani
05-14-2018, 8:14 AM
lol

im having issues with my brake light now, everything is wired up, but when i hit the brake pedal, the lights get dim(headlight and tailight) - they get dim and then the circuit breaker pops. no lights. let off the pedal for couple seconds and breaker resets, and my lights are back. its at the brake light switch, i undo the wires- Red with yellow strip and it doesnt popped the circuit breaker- but i dont have a BRAKE light now- just a running light. im thinking theres a short on the other wire coming off the switch- orange with white strip.

Tattooo
05-14-2018, 8:26 AM
lol

im having issues with my brake light now, everything is wired up, but when i hit the brake pedal, the lights get dim(headlight and tailight) - they get dim and then the circuit breaker pops. no lights. let off the pedal for couple seconds and breaker resets, and my lights are back. its at the brake light switch, i undo the wires- Red with yellow strip and it doesnt popped the circuit breaker- but i dont have a BRAKE light now- just a running light. im thinking theres a short on the other wire coming off the switch- orange with white strip.

Yep you have a short all right..... It won't be hard to find get a test light....

farmall
05-14-2018, 10:33 AM
Remove the light bulb and disconnect at switch then ohm check that wire to ground. (Battery negative cable should be disconnected for ohms checks so no smokee da meter fuse while fiddling).

Inspect bulb and socket because vibration ain't kind to either.

Ohm brake light switch contacts to the grounded shell with wires disconnected and brake applied to ensure switch doesn't internally short to ground.

ChopperDani
05-15-2018, 7:05 AM
yulp.

easiest thing i was gonna do was jump the switch, just to rule it out. but im pretty positive the orange with white line wire is shorted out somewhere. i have a running light as of now, ill find time this weekend to trace down the wire. its all tucked in the oil tank under the seat.

no brake light wont stop me from scootin around

Tattooo
05-15-2018, 7:25 AM
i have a running light as of now, ill find time this weekend to trace down the wire. its all tucked in the oil tank under the seat.

no brake light wont stop me from scootin around

No sir a light won't stop you but a dead battery or a fire sure will...... Good luck...