Inconsistent / spongy front brake-update
 

Hi, I have a 2005 cbr 600f and for a few 100 miles now I've had an issue with an inconsistent front brake lever, in that every so often I will pull the lever and I get 10mm move travel before I get any brake. Then after a bit the brake is fine again. Regardless when the brake kicks it it works but the additional travel is some what unsettling.

I have tried the following:

> Bleed both sides by opening and closing the nipples while pumping the leave. On both sides I got some air out. initially the brake was better but after 50 miles the inconsistent lever came back.

> Then took it to a garage who checked the discs for distortion which it had none, and he also vacuum bleed the system and bleed at the mc also. Again it was better but for only around 20 miles

> A couple of weeks ago I rebuilt the master cylinder and then back filled the system with a syringe running 2 to 3 cycles through the mc. I then bled the system opening and closing the nipple while pumping the lever, this would for the first 3 or 4 pumps release some air bubbles in the fluid then it fluid appears air free. I've followed by doing the same to the other side and by the time I get back to the original side I'm getting air coming out again. I managed to get both sides air free and took it for a short ride and straight away I'm getting the inconsistent lever again. I tried to bleed the system again and yet more air and to be honest on the left calliper it seemed to be a flow of very small bubbles suggesting air is getting in but from where?

I tried leaving a cable tie in the front lever over night but this did not make a difference. It seems that when the bike is left in the garage when I first squeeze the brake the lever will go all the way to the handle, and generally if I'm braking a lot it seems fine but if I don't use the front brake for a couple of miles it's rubbish again for the first squeeze.

Has anybody come across this problem? I've done some research sony next step would be I guess to strip and clean the callipers. I did take the dust guard of Sunday and I could see the pistons were pretty dirty and gunked up so could it be possibly a piston is not free to move?

Thanks for any advice.

Sam

Classic bad master cylinder. Get a new one.

I've rebuilt the master cylinder and to be honest I couldn't really see signs of damage on the seal or internally within the cylinder. The diaphragm did look a bit tired but I'm not sure if this would have an impact?

Update:

Today I have rebuilt the calipers and also upgraded to hel braided lines. However I'm not having a pita bleeding the brakes.

I've get some pressure on the lever and when I open the nipple and fluid flows out and I close the nipple. Then I have zero pressure at the lever and I have to pump it maybe 5-10 times to build pressure before opening the nipple again, surely this is not correct? When I do get pressure I open the nipple again air is coming out so I'm going around in circles. Any suggestions? I'm leading towards air getting sucked in when opening the nipple or the nipple seal itself. Has any used Teflon/ ptfe tap on the threads to get a better seal?

Try reverse bleeding the brakes with meat injector or similar device. Should help a ton. Just my .02

I tried to reverse bleed it and initially this was work but then once the system got pretty full the pressure built up and now it mainly leaks out of the bleed nipples.

I think I'm going to try some Teflon tape or paste and also put a bit of grease around the nipple and have another go at bleeding.

Is the nipple leaking or the threads for the nipple? Brakes are super simple, it shouldn't be this difficult and you should never use teflon tape or a sealer grease in the system.

It seems to be leaking from the threads. I took the nipples off and gave them a through clean up and put them back on with some grease around the outside. I was able to bleed the system and get a really firm lever.

On a 40-50 mile test ride the lever was solid and really firm for 90% of the time but still occasionally it was inconsistant and had more travel before the brakes worked. When the lever is working well the brakes were solid and much better than when I had standard lines but it's just the times when I get more travel in the lever that worries me.

Any suggestions? Still got some air in the lines maybe? Or a fault with the MC still?

Sam

I'm still going with the master cylinder. I would start with a new[even new to you, a known good used one] master and go from there. They are cheap on ebay and local craigslist's. The fact it was leaking from the threads tells me either thread damage or bad bleeder valves. They could be letting air back into the system as you use it....

I think that will have to be my next course of action, any suggestions on an upgrade while I'm at it?

The only other thing I can think of is that I did get the front wheel changed recently and after around 200-300 miles is when I started getting this issue originally, any chance this could be related?