Total Motorcycle Community Forums

Hi,
Last item on the agenda in my restoration of a yamaha '78 xs400: the brake caliper pistons seem to be stuck. The master cylinders have probably been devoid of any brake fluid for a while, and the previous owner had detached the calipers to keep them from seizing on the brake rotor. but now the brake fluid seems to have from this crust between the piston and th caliper cylinder, and seems to be very badly seized.

Any ideas as to how to pump the piston out?? Im assuming the first thing to try is to rebuild the master cylinder, refill flluid and pump the brakes, but in case this fails I'd like to be prepared. The caliper assemblies are great, might need a caliper rebuild kit, but Im hoping I can save the piston itself. Any help is appreciated.

Thanks in advance.

That crust holding the pistons stuck is probably corrosion from electrolisis between dis-similar metals. If you can't push the pistons out with the MC after you fill and bleed the system, you can try pushing them out with a greasegun after you put a zerk fitting in the brakeline hole in the caliper. Then you will need to clean and hone the bores and install a rebuild kit. Should be cake after that......

now the zerk fitting goes in where the banjo bolt is supposed to be right?

I might be missing something here, but Now if I rebore the whole thing, how are stockpistons going to fit correctly??

Thx
-K

lubes like Kroil (www.kanolabs.com) or rustreaper (www.rustreaper.com). I know these arent for normal use with braking systems, but Im going to have to flush the system with brake fluid after rebuilding the calipers anyway....

I have used kroil with some success in the past on parts for my car. Not sure if the same wisdom applies in this case.

Hold off a minute. Take the thing apart first and figure out what the damages are. Reason i'm sayin is because i don't think they make a full rebuild kit for your brakes (if i remember, they no longer make pistons for it)

you might have to go with an aftermarket or system off a junked bike

If the MC won't push them out, yes the banjo mounting hole . But the MC should be more than enough to push them out.

Hone does not equal bore. Honing just smooths the surface and cleans the bore (hole) up, without removing much material.

Can I clean up any surface rust using fine grain sandpaper? I guess this will just remove the rust off the piston and cylinder... and not any significant amounts of material.

If the pistons are pitted, you're most likely going to need new pistons (if they make them) Even if you clean the rust off, the pits left behind (if any) will eat up the piston seal and cause the fluid to leak. Just make sure before you go to all that trouble!

I do know of a website where I can order pistons. so that shouldnt be a problem. Just that I would to make sure before going out and spending $60 on new pistons. Also, I have access to a lathe where I can machine my own pistons.

Does anyone happen to know, what the best material to machine the pistons is? Im assuming they're polished aluminium... am I right??
Just a thought, Im sure its easier to just go out and buy the damn things.

-Kartik

OK,

So i got my Pistons out. Rear piston is still good, has one small tiny spot on it, but i think its still good. The front pistons badly pitted, and will need to be replaced.

Both pistons are thte same size. I noticed however that when they are pushed fully inside, they sit lower in the Rear Caliper cylinder, but stick out about 2-3mm more in the front caliper cylinder. Is this normal?? I know the rear brake should provide only about 30% of the braking power, so is this solved by letting the piston sit deeper in the cylinder??

-Kartik