Help rebuilding rear brake caliper and bleeding rear brake.

I recently bought a 1984 700f. I've been doing a tune up because it sat for a while and one of the problems with the bike was that the back brake would stick engaged. I was told to pull out the pistons and polish them and if need be, to rebuild the caliper. I tried to push the pistons out using the rear brake to no avail. I thought I may need to bleed the rear brake because I had previously taken the caliper off and put it back on without bleeding it. However when I try to bleed the rear brake no pressure builds in the line and the fluid just dribbles out of the bleeder. I was told this could be my rear master cylinder?

Can anyone help me out?

 

You probably will have to rebuid the master cylinder and change the piston and dust seals on the rear brake. Check to see if the piston does not have rust pits in it when you do this.

 

Alright thanks. If it does have rust pits is there any way to repair them or do I have to get new pistons?

 

Also replace the rubber brake line..when you apply the brake it pushes fluid to the caliper and when you let off the line will suck itself closed from the back pressure if its bad..ive had this happen on bikes and cars..new line and its fixed. I went with steel braided on my 84 vf700. Find a local hose maker and they will make one for you...

 

If the pitting and rust is bad you will probably have to get new pistons. Depends on how bad though.

 

Nothing you stated suggests that your brake master cylinder needs rebuilding, but air trapped in the lines will do what you say, and MAY only need a good, proper bleeding to set things right.

Often, just getting the pistons oot and cleaning them--along with the rubber seals in the caliper--with some green scotchbrite will free things up and fix it; otherwise you should replace both caliper seals.

 

Although the pistons can and do stick a more common problem is the caliper mounts are stuck. These mounts are supposed to be free allowing the caliper to move from side to side as the pistons are on one side only. The caliper has to float after a period of time the grease in these mounts dries up and stops the caliper from floating. As for bleeding keep trying with both pistons retracted and if need be remove the caliper and raise it up to allow air too self bleed.

 

thanks everyone. I ended up pulling the pistons and sanding them down with 600 grit and steel wool. The seals looked fine. I also replaced the brake line that runs from the reservoir to the master cylinder. The brakes bleed fine. I have yet to take it on the road though. It looks like the pads are touching the rotor when they are retracted and it feels this way as well. Do i have to actually used the brakes while riding to seat everything and work it all together?

 

Odds are your seals are pretty dried up if they are original, I highly recommend changing the piston and dust seals even though they may look OK. The seal helps to retract the piston slightly and if they are old and dried up they will not retract the piston properly. The slide pins should be clean and well lubed also.

 

Which pin is the slide pin?

 

I had this same problem after my VFR 750 sat for a few years. I was surprised that even after I completely rebuilt and bled the system, the brakes were still binding. It turns out there was some oxidation on the surface of the pads and rotors which prevented them from turning freely. I took a chance and rode it anyway. The oxidation wore off after a couple of miles, and the brakes now work without any problems.

Besides the great advice from the other posters here, I have a simple suggestion (because I made this mistake). Before bleeding the brakes, be sure to replace the rubber boot in the reservoir. That boot prevents air from getting sucked into the system as the fluid level drops. It is very difficult to bleed the brakes without assistance unless that boot is in place.

 

The caliper slides on both the bolts holding the caliper to the bracket on both the front and rear. The upper bolt 14mm wrench size has plain length on the end of it and fits into a hole in the bracket with a rubber boot to keep dirt out and grease in, with the caliper off you should be able to slide this bolt in and out of the hole in the mounting bracket freely.
The lower bolt 12mm wrench size passes through a steel sleeve in the caliper itself when you look at the caliper you will see a rubber boot on both sides of the caliper the sleeve should move in both directions freely if not drift out the sleeve and clean then lube, the rubber boots stay in place until after the sleeve is removed so when assembling install the boots first then slide the sleeve in.

 

auxpowerunit: Thanks for your reply. I don't think its the oxidation being that I replaced the pads. I still need to take it for a spin and see if the new pads wear in. Sorry if I didn't mention the new pads earlier. Also the rubber reservour seal is still in place, though it fits loosely. I was able to bleed the brakes fine though once I replaced the line running from the reservoir to the master cylinder.

Bikeboy: I appreciate the detail and it will definitely be helpful. One more question before I take the pins out. What kind of lubrication should I use. Heat resistant grease?

 

Believe or not unless you were one of the country's top racers there is not much heat in this area a good wheel bearing grease would be all you need.