front brakes "clicking"/"thunking" - 1980 gsx750

[CincinnatiKid]

I was never enamored with the brakes on this bike when I bought it 2 years ago. Not responsive enough for me... So a few weeks back, I finally pulled the trigger on SS braided brake lines (Venhill USA) for the front and rear brakes. The rear brake feels great, but the front actually feel worse/squishier than before. Not good!

1) the lever feels like it can go about 50% of it's total travel before it even starts to exert any braking power.

2) I was able to do some bootleg fix to it by shimming the space between the lever and the M/C plunger. It has been a temporary fix that has worked for a few weeks now.

3) spent a few hours trying to bleed the system when installing the new lines, and hit a point of diminishing returns, where we are now. Spent an hour today trying to bleed the system again, no improvement.

4) In the past week or so, I have noticed a "clunk" sound coming from the front caliper(s) when the brake lever begins to exert braking power. only while moving though. As if the pads grab the rotor, and there's free play in the caliper mounting, and they click against their rigid mounting. not sure if that makes much sense. It's very hard to localize the sound, and hard to replicate in the shop with the front wheel lifted. I took the calipers off today and tried to find something abnormal and couldn't.

frustrating, and unnerving!
should i totally drain/flush/rebuild the front brake system?
Should mention that the calipers were shot when i bought it and I rebuilt them with all new seals etc.

 

[tom203]

Might be a warped disc(s)...the old calipers got used to them, but rebuilts haven't

 

[Brendan W]

Could the slider pins be worn ?
Is there a rubber sleeve on one of them ?
Some bikes had this. Check the fiche.

 

[bwringer]

Sounds like you need to bench bleed the master cylinder. There's a wee bit of air caught in there that you can't get with vacuum bleeding or pumping.

Loosen or remove the banjo bolt so fluid can escape the master cylinder. Put some rags around it to absorb the fluid that will come out. Remove the brake lever and your shim, and using something blunt to slowly push the piston in alllllll the way. A bit of air will burp out. Repeat a few times then reinstall/tighten the banjo and vacuum bleed normally if needed.

A certain amount of clunking is completely normal. Make sure the sheet metal shims and shields are still in place. The clunking seems to increase as the brake pads wear, so consider replacing the brake pads if they're worn.

 

[Gorminrider]

4) In the past week or so, I have noticed a "clunk" sound coming from the front caliper(s) when the brake lever begins to exert braking power......

Just possibly your stanchions, not your calipers...a worn bushing on the stanchion tube?,( if you have that type)...anyways check the fork itself if it's definitely nowhere else.

 

[CincinnatiKid]

awesome, thanks guys. gonna tear into it now, do a full re-bleed and that master cylinder bench bleed.

the slider pins for the calipers are in good condition, fresh grease, and with their rubber dust boots intact. probably will clean and regrease them while i'm at it today, though. new pads are on the way.

rebuilt the front forks a few weeks ago with new seals and fluids. stanchions and the rest looked good, but i'll keep my eyes/ears on it.

 

[CincinnatiKid]

bench bled the M/C, then bled lots of fluid through the lines. bled at the M/C banjo fitting, took the shim out.
brakes still suck.

i installed one of those hydraulic brake switches at the T-fitting, and it doesn't light the brake light until the lever is more than 2/3 traveled.

I've used a vacuum bleeder, the traditional squeeze-lever/open-bleeder method, and then gravity bleeding. none can seem to find the air bubble that is in there. i think next step is going to be disconnecting all of the lines and reinstalling the whole system fresh after blowing compressed air through the lines.

 

[Steve]

I got a set of stainless lines about a year ago. Installed them and never could get proper pressure on the front. Noticed that not only did I not get pressure, the lever moved if I held it. Found a leak at one of the crimped fittings. Put the original rubber brake hoses back on, full pressure is restored. I have since gotten another set of stainless lines, waiting for my driveway to be installed so I can get back to working in the shop.

.

 

[CincinnatiKid]

curious, definitely interested to know if your second set works out.
i cant find any leaks or manufacturing flaws in my lines.
tried bleeding again yesterday, didn't help.

i went back to adding the shim/pre-loader between the M/C and the brake lever. it works, but i don't like that it's the best performing option at this point.

wondring if my master cylinder is leaky or gouged on the inside, and after the piston makes it past the gouge, it retains/exerts pressure to the calipers.

 

[hillsy]

If you use a syringe at the bleed nipple, you can push / pull fluid up and down the lines as many times as you like and this will be the best way to ensure you get rid of any trapped air.

Then when you button it all up, turn the bars to the left, pull the lever in and wrap an octopus around it to hold it to the bars. Leave it overnight then try it the next day. The pressure you leave in the system overnight will help any air bubbles rise up to the MC.

 

[CincinnatiKid]

thats a good idea. now to find a syringe. amazon!

 

[Gorminrider]

pet stores or feed stores have fairly big ones. failing those,just maybe the Pharmacy too

 

[TxGSrider]

If you use a regular syringe (or a turkey injector syringe works pretty good too) you'll need to work pretty fast as the plunger can start to swell after a little while and make your work a little difficult.

 

[Gorminrider]

good point^^^ and best to dissassemble and clean the syringe well with soapy water to re-use too. If you can't flush from bottom,(never worked all that well for me using syringe) a syringe can be useful to suck out air bubbles from the top at the Master Cylinder's ports...the little tiny relief one can be a nuisance other wise but sucking them out can make the difference rather than trying to move air from top to bottom against nature..

But! given your 50% squeeze, it might also be useful to check if you have original Master on the bars...I know from experience that the wrong Master's bore size affects this lever throw....not sure about your bike but if varying models came with 2 disks or one disk, there might be a diff at Master too...?someone maybe swapped in the past.

 

[CincinnatiKid]

true, i thought about that as well... hard to determine if the M/C is OEM or a swap. Looks the same shape as the one from the parts fiche, but obviously that can be deceiving.
I believe that the bore on it is (rather, should be) 14mm, which seems to be the aftermarket standard for a dual disc setup.
Since my front end has gone away from a stock direction, I was looking at just swapping the M/C for a new one if there was a good choice. Ebay has lots of cheap chinese options, but I also like the feel of the Kawasaki Ninja series levers. I put some on my sportster recently, and they just feel good in the hand. I'll probably keep my eyes open for a reasonable set.

 

[salty_monk]

After a bleed I always cable tie the lever back to the bars (or as far back as I can) and leave it at least overnight. For some reason the extra pressure seems to clear out any air in the system.
I've used the same trick on cars before too...

 

[CincinnatiKid]

still fighting. used a cable tie around the lever overnight, and it seemed to fix it, until i actually began riding, and it all went back to regular (lack of) pressure.

shopping around for replacement master cylinders... I think the M/C bore size for the 80 gs750E is 14mm. anyone know off the top if that's correct?

thx!

 

[CincinnatiKid]

still fighting. used a cable tie around the lever overnight, and it seemed to fix it, until i actually began riding, and it all went back to regular (lack of) pressure.

shopping around for replacement master cylinders... I think the M/C bore size for the 80 gs750E is 14mm. anyone know off the top if that's correct?

thx!

okay i believe i found that it's a 5/8 bore. according to a picture in the manual for the '83.