Rear brake master cylinder questions

[billyjakester]

I have a 2000 Valk Std. and installed new pads on the rear.  I removed the caliper, cleaned it and replaced both seals on each piston.  Polished both pistons and reinstalled the caliper with new Honda pads.  Started bleeding with a MityVac and could not get pedal pressure and I had no rear wheel brakes.  I assumed the pistons did not activate.  Next, using my SpeedBleeders, I bled the system again and ran almost a quart of new brake fluid thru the system.......pedal travels full stroke, but still no brakes.  Finally removed the caliper, leaving the brake line attached.  Placed a block of wood in the caliper to simulate a rotor and pressed on the rear brake pedal to activate the pistons.    The brake pedal had full travel, but the pistons did not budge......Nothing.....

My guess is the master cylinder seals are shot and the brake fluid is merely cycling around in the master cylinder and not building pressure to be delivered to the rear brake pistons.  I assume I'll need to tear down the master cylinder and replace seals, etc.  The bike has about 64k and I've changed out both front and rear brakes previously without any problem. 

Anyone care to chime in with comments, suggestions ?

thanx,
bill

[sandy]

I have 149K on my '98 Tourer. No troubles with any cylinders. All are original. I use to use a MityVac with no problems. I've upgraded to a bleeder that uses an air compressor.
I think you have an air bubble that won't move. With some fluid in the master, pump the pedal all the way through it's travel, but vary the frequency. Get pretty vigorous to bump the bubble. Watch for bubbles coming back up in the reservoir. Good luck.

[Louis Durocher]

be sure the master cyl is full
1- apply brakes
2- open the bleeder with the brakes still applied
3- close the bleeder with brakes applied
4- take your foot off the brakes
5 repeat the 4 steps untill no air is coming

Be sure that all is clean in the master before to attemp to bleed
Use new DOT4 brake oil

[Brian]

Order the OEM Honda rebuild kit and go for it using new dot 4 fluid. I did it following the shop manual for cleaning and rebuilding. This took care of my long brake pedal travel issue before I lost rear brakes. I only have 25,000 miles on a 97 standard. I found a gummy substance on the spring side of the piston and a film of aluminum half way covering the piston seal. Rear brake working like new now. It was an easy fix and well worth it.

[Pappy!]

I just did the same thing you did.
The MityVac sucked the piston far enough forward in the master cylinder that it is now stuck in the cylinder.
Remove the banjo fitting at the front of the cylinder and use something like a 1/4" extension to free the piston back up. Stick it in from the front and pop it with your hand. It will immediately come free. Re-assemble and bleed using the pedal itself instead of the MityVac. Check to make sure the pedal stays steady.
If you take it apart and free it up (like I did) you will see that doing it the way I described above will not damage anything inside. Of course if you want to then just disassemble it and clean it and reassemble for peace of mind. I ordered a kit after doing this just in case but mine is sealing perfectly still.

[knockdolian]

I have a lot of experience with many bike brakes. I have found when overhauling brakes if the parts are left dry for a time. IE no fluid in the master cylinder the seals can dry out. Assuming you didn't do that, undo the hose at the master cylinder and put a finger over the hole. Operate the pedal while pressing on the hole.  As you pump any air will be expelled. then the pedal will go hard. This will prove the master cylinder and get any air out. Then do the same with the hose on holding the calliper end. You only need a small amount of air in the master cylinder to cause problems.

[da prez]

 One often over looked problem is to be sure the master cylinder is going to the fully returned position.  If it is not , then the fluid will not get in to do it's job .  Pull up just slightly on the brae pedal  and see if the piston linkage pulls out of the cylinder a little. Wear and dirt can cause this to happen. Back to basics. Start with the adjustment checks of the mechanics before going to the hydraulics. I see this a lot on bikes brought in and lately on new parts not adjusted or finished properly.

                              da prez