rebuilt master cylinder wont build pressure!

83 xj 550. so over winter ive gone through a lot of the bike and have now gotten to the routine maintenance of the braking system. started with the issue of the caliper would seize while pushing the bike in reverse (thought it was kinda weird, as did my mechanic). So i go to bleed the brakes and realize the bleeder valve is gnarled, replaced it for $1 from a local shop. go to try and bleed them and nothing happens. no pressure builds and nothing comes out the bleeder valve. order a rebuild kit for the master cylinder and caliper. before disassembling the m/c i disconnected the banjo and brake line at the m/c. pump brake lever and no brake fluid come out of the master cylinder. rebuild master cylinder (triple checked to make sure seals are in the correct spot and facing the correct direction), reinstall on the bike and still the same thing happens with no pressure being built or any brake fluid coming out of the m/c. take it off again and check the seals to make sure i put them in right and test the vacuum by plugging the banjo hole with my thumb and the two holes in the master cylinder and had my brother pump the brake lever, built a little pressure and when i released the two tiny holes inside the master cylinder i could hear and feel pressure being released so i was fairly positive the boring of the cylinder isn't distorted and that the rebuild kit is installed accurately.there has been numerous beautiful days i have already missed riding this spring due to the xj not being ready. Please help me loose my winter training wheels and get the CAR back in the garage!

 

I'd kinda figure that the seizing caliper and pressureless MC kinda counteract each other, but what do I know.

Please tell me you have new brake lines. Those 2 little holes in the MC are tricky, and need to be totally spotless before the rebuild. Also, our MC really doesn't build up a whole lot of pressure anyway. If you lost all your fluid, it would take forever to pump fresh stuff from the MC to the caliper. I tried bleeding manually and gave up after about 30 mins. Used a vacuum pump to draw the fluid to the caliper, and the sucker was bled and operating correctly in about 45 secs.

 

This sounds completely nutso, but you have a little air bubble at the top of your master cylinder, right where the brake line connects.

This takes a lot of patience, but what I did and have a rock hard lever, is with the reservoir half full, so all holes are covered, unbolt it from the handlebar, and DON'T spill it!!

Tilt the master cylinder a little, so the part that goes to the brake line is a little lower than the handle end.

That little air bubble with show up, either through the return hole, or if you slightly pump the lever, it will pull through on the piston return.

Once you got all that air out, mount the master cylinder back on the bike and bleed the rest of the system, making sure no air gets back in the master!

Your brake will be firm and reliable!

 

Couldn't you just twist the handlebars to change the angle of the master cylinder? Instead of having to unbolt it.

 

well i dont know wtf the problem was, but instead of using a vacuum pump i just used a clean piece of tubing and sucked on it until i saw fluid coming through. got everything bled and functioning, the pads are still too tight though. I tried depressing the piston into the caliper with a bar clamp and then bled a little that way. but they are still not completely clearing the rotors. any suggestions?

 

They shouldn't completely clear the rotor. When they grip the rotor, nothing should move. But when you release the brake, the pads are still insanely close to the rotor, but they don't grip it. If the brakes are still activated when you release the handle, either the free play screw needs adjusting, or the smaller hole in the MC is plugged. Take a tiny piece of guitar string and use that to clean that one.

 

You need to replace the caliper seals. Also make sure that the channels they sit it are spotless.

 

Yeah no kidding. You actually need to completely rebuild the caliper. The 550 has a "holding block" style caliper; it is two pieces: the part that bolts to the fork leg (the holding block) and the caliper itself which has to be able to slide freely and smoothly in relation to the holding block.

You need to take the caliper apart; clean the seal channels and replace both seals; polish the piston and inspect for pitting, replace if necessary; clean the bore in the holding block and inspect both it and the holes in the caliper for wear; polish or replace the sliding pin and probably the pin boots (they're usually shot) and carefully reassemble with new pads of course.

The diagrams in the manuals were lifted from the fiche and have some inaccuracies/omissions, here is a corrected diagram. Note that this is a RH caliper (Seca) yours is an identical mirror image.

 

I did try to tilt the bars, but did not enough tilt, to move that air bubble.

Whatever works, I guess. I can almost do a "stoppie" now!