I'm having front brake issues on my 08 TXC250 and now it seems like the front brake master cylinder is not pumping fluid ... Has anyone ever replaced any parts on one of these MCs or at least tore them apart? They look very trivial with nothing more than a rubber boot as the only entry point into the device other than the banjo hole ... Here is what happened ... I lost a screw from the MC and lost all fluid ... then when I added fluid and tried to bleed the system, no luck ... I removed the banjo screw from the cylinder and tried pumping the lever with the screw out and got very little juice out the banjo hole so I'm thinking the MC is not pumping fluid ... What is the behavior of the MC in the case of the banjo screw removed?
The master cylinder isn't a pump as such, so what you describe is normal. The cylinder is connected to the reservoir by a tiny hole called the relief port (that's what we call it in the UK anyway!). As soon as the lever is pulled the piston moves past the port and the master cylinder and caliper assy are sealed from the outside world. The port reopens when the brake is fully released, but as it's a small drilling then you won't get fluid pouring through there. That's why if the master cyl. is not primed (filled with fluid) it won't pump - there are no valves to direct fluid in the correct direction. First, refit the hose to the master cylinder (check the condition of the sealing washers and if they're marked then replace them or at a push you can fettle them with some fine wet and dry on a flat surface). Now to bleed the air out: Hopefully the lower part of the system will still be full of fluid. Take off the caliper, remove the pads and clean the exposed parts of the pistons as best you can with a toothbrush and brake cleaner or gentle detergent and water. Push the pistons back into the caliper while holding the caliper with the hose connection uppermost - this will chase out the air and with luck you'll see the reservoir start to fill with fluid. If so, refit the pads and caliper, top up the reservoir with fresh fluid, bleed conventionally to get rid of old fluid and any remaining air and operate the brake several times while continuing to top up the fluid level to reset the caliper pistons. Mebbe it's still spongy, or there wasn't enough fluid remaining in the system to push back into the reservoir. Time for reverse bleeding. This involves injecting brake fluid into the bleed nipple until it appears in the reservoir. You''ll need a couple of 50ml syringes (one for the bleed, one to remove excess fluid from the reservoir), a short piece of flexible tube that will fit the nipple and the syringe, plenty of fresh brake fluid and something to clean the inevitable spills off the bike's bodywork... 1) Do the remove caliper / push the brake pistons back / refit caliper thing again to chase the air out of there. 2) Attach hose to syringe and fill syringe from brake fluid container. 3) Fit the hose on the nipple (spanner on first!), hold the syringe plunger side up and tap it a few times to dislodge any bubbles that may be clinging to it's sides, open the nipple and then steadily push the syringe plunger down until air free fluid appears in the reservoir. It's a lot easier if you have a buddy around who can clear excess fluid from the reservoir, if necessary. 4) Perform a conventional bleed to chase any remaining air out of the hose and to prevent it finding it's way into the caliper. 5) Pump the brake to reset the caliper pistons while topping up the reservoir as necessary. Check brake operation and repeat steps 1-4 if necessary. Brake bleeding is a bit of an art and there are all kinds of little tweaks to get that last little bit of air out. Dirt bike brakes are the worst, with that long line that arches above the master cylinder. I'm sure that other forumites will chime in with their own techniques. Usual disclaimers apply! If you are in any doubt then get assistance from a qualified mechanic.
ok I'll try all this and thanks for the info and brake operations ... The cable to the caliper is dry and I had already tried pushing my pistons closed so they are in the caliper now ..
That's ok, just start with the reverse bleed procedure - you'll probably have to do it a couple of times as any little bubbles that find their way into the caliper can be hard to budge. Slow and steady is good. take your time and let me know how it goes.
I can't find my syringe so I'll have to get another one tomorrow ..... the normal bleeding process just goes nowhere .. ;( .. It pumps a small amount of oil and air out the bleeder and this is all it does after numerous attempts ...
In an emergency you can sometimes get it started by using the hose banjo on the master cylinder like a nipple - slacken the banjo a touch, squeeze the brake, tighten the banjo, release the brake, squeeze the brake rapidly a few times (you may see bubbles appear in the reservoir). Repeat 'til the master cylinder is primed, then bleed conventionally. It's messy though and requires an everlasting supply of brake fluid. Best avoided if you can wait 'til tomorrow.
I think I got it bled finally ... Along with the reverse bleeding, I had to bleed at the bottom banjo bolt a couple times and conduct the entire process 2-3 times to get the brake back ....I might have introduced a leak by the bottom banjo bolt on day one that added to the length of times it took to get all the air back out ... No test ride today but that will happen early tomorrow ... Thanks for the help ...
