The front brake on my KLX gets bled with new fluid and has a firm lever. Then after a couple of days the lever progressively gets softer, nearly unusable after a week. The fluid turns dark grey almost black after only a week also. This is Lucas DOT4 fluid.

The important part here is that when the brakes are bled they perform great, then progressively get worse. I know a lot of riders have trouble getting a proper bleed but that's not my case.

The bike has new caliper seals, new bleed nipple screw, new oem brake line, new master cylinder seals. I also added a banjo bolt with nipple bleeder at the master cylinder. I have checked the pads, banjo bolts, nipples and brake lever and it has no fluid leaks at all.

Has anyone ever dealt with something similar and been able to find the cause? It's baffling to me that a firm brake lever with no leaks gets softer and softer, then I have to bleed it again to make it work. I have been dealing with this for over 6 months now and I am one step away from throwing the whole front brake system away and getting all new parts from a better system.

 

The more interesting part to me is how quickly your brake fluid is turning dark...and not just dark...but gray. Most DOT 3-4 fluids will get an amber tint after a bit and then get almost black over a good deal of time. All your other symptoms still sound like a bleeding issue. You can get what feels like a good bleed with good lever pressure, but if an air bubble was in a less critical space in the system, it can migrate to a more critical spot in the caliper or master cylinder. Yes, usually you get a good bleed and it remains that way, but occasionally that stray bubble(s) can be a PITA. In your case, however, I'm a little wary of the reason your fluid might be getting discolored so rapidly. Something in the system may be breaking down slowly to degrade your hydraulic pressure. Usually this is indicated by a complete failure, but not always necessarily so. Were you having any brake issues that prompted you to change fluid and bleed the system or was it only a basic service? How long have you had the bike and are you the first owner? Sometimes people can use the wrong brake fluid which can rapidly or slowly degrade the system, and just changing won't always undo the damage.

 

I replaced the fork seals last spring and stripped one of the caliper bolts. Instead of taking the whole bike to a shop to remove the bolt, I remove the brake hose from the caliper and took the fork leg and caliper to the shop. After this I replaced all fluid and bled. Been at at it since then. From that day to today I've bled the bike close to 10 times if not more, trying to improve on the bleed process each time. Last time I reverse bled from the bottom, after filling the caliper with fluid first. I got 3 MCs of fluid thru it with the syringe then bled at the bottom nipple, then at an MC banjo bolt nipple.

I agree that it's strange that the fluid is turning dark grey so soon, specially with no leaks anywhere. I don't ride the brakes hard at all, I don't ride the bike fast enough to even cause them to fade or overheat.

What else could cause the fluid to turn dark so fast?

 

I would take a close look at the bore on the master cylinder. It doesn't take much pitting to allow it to suck air into the housing. My 06 GSXR would exhibit the same symptoms(except for the brake fluid change) and it turned out to be caused from pitting in the bore. Suzuki ended doing a recall on most of the gixxers from 05 on to replace them.
Air can only enter from the m/c or the caliper so a quick method to isolate would be to see if the caliper burps air when you first try to bleed the system. If there are no bubbles and you have to pump a few reservoirs full of fluid before you get a firm lever then I would suspect the m/c.

 

Try pushing the fluid up from the calliper. A rubber hose over the nipple on the calliper and the other end to the pump out of your wife's Favorite hand cream. ( clean it first). Then submerge the pump pickup in some fresh clean brake fluid and pump away. When no more air bubbles are seen simply turn the bleeder screw down install master cyl lid and clean up.
If that doesn't work and you keep loosing prime Id be hedging my bets on a damaged master cylinder.
My experience as a automotive/commercial transport mechanic has taught me it doesn't take much to damage the seals in a master especially during bleeding. Bleeding using the mastercylinder as the pump can damage rubbers just by simply stoking the piston past the normal range of travel into foren debrise and buildup in the master cylinder bore. Some times an o/haul kit will do the trick and sometimes it won't take. Maybe the rubbers are damaged from bleeding full stroke. You don't even have to see damage like scoring and gouging to cause a leak, air or brake fluid it can go both ways.
Good luck

 

Thanks guys. I did use a syringe and rubber hose to fill the brake system from the caliper nipple. I put a few reservoirs thru it to make sure there were no air bubbles in the line.

I did rebuild the MC with new seals in Aug, 14, but it makes sense that could be the problem. Before putting the brake line on the MC, I place my thumb over the exit hole and pump the lever, by the 3rd put I could not keep the fluid from squirting out. I took this to mean that the MC was was working properly. Was I right or is this test meaningless? Either way, I guess it would be a good idea to take apart the MC for inspection and cleaning. Last time I did that there was a lot of dark gunk on the plunger, but I cleaned it off real good before replacing the seals.

 

What ever you do, do not drink the grey fluid, did you see the movie Prometheous?

Honestly I don't know what but it sounds like your system has become infected. No help here.

 

jajaja, good one.

On my list to do:
disassemble, clean and reassemble MC
disassemble, clean and reassemble Caliper
check caliper to make sure the bracket is not bent or off center (I have already replaced the front wheel bearings and had the rotor checked to make sure its not warped)
wrap top half of bleed nipple screw with teflon tape before bleeding

 

neon, does your bike still have the rubber brake hose? I have seen the inner material break down on some of these in the past and cause discoloration. It's not real common, and it doesn't always give itself away by a bulge or split in the hose. Have you inspected the hose closely...I mean really, really close.

 

TNC, it does have the oem rubber hose but it's not the stock one. It supposedly came off of a 1500 mile SF. I haven't inspected it very closely but it looked almost new. However the hose has only been on there for a month and the problem started 9 months ago so I don't suspect it. Should I?

The caliper seals were oem, but the MC seals were all *****. Could they be deteriorating after 6 months?