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wierd rear brake problem

  • Thread starter Thread starter halz426
  • Start date Start date
H

halz426

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So I just rebuilt my rear master cylinder, bench bled it and then bled the lines.

Here is what I don't get >>>>>>> When I apply the rear brake the calipers will not diss-engage, unless I crack open the bleeders, then they release.

It seems like it is not bleeding back. I have triple checked that the springs & plungers are installed correctly, checked the clymer manual and online diagrams.

This is for a 77 GS70 with a K&L MC Rebuild kit

Any input would be appreciated as I want to get riding and enjoy the new bike...

Thanks
 
Well for one, with a part like this OEM would be the way to go. Even so, you shouldn't be having this problem with an aftermarket one. I have heard of this happening when there is a tear or break inside the brake line where fluid is allowed to travel one direction, but when it goes the other way the broken flap jams itself and doesn't allowed fluid to pass. We're the calipers rebuilt too? Was this happening before? Even if the line isn't the problem it should definitely be replaced as well. That's all I can think of, someone else will probably chime in with more.
 
Spring goes in skinny end first. has that metal hat looking thing in the end of the spring..that goes in first.Then the rubber cup goes in with the cup facing the spring..spring end fits into the cup. Lube the bore with clean fluid before assembly.

Next the piston goes in with the metal starred end facing the flat side of the rubber cup. Then the drive rod goes in and the ball fits into the cupped end of the piston.

How hard did the pistons go back into the rear caliper halves? They should go in snuggly but not so snug that you cant push them in with your fingers.

Did you clean the grooves the seals fit into REAL good and be sure the seals dont have any scale on them as well? Or new seals as well?

Did you remove the brass nipple that resivoir hose fits to and see that the 2 holes in there are clean and open?

If its assembled right, the caliper pistons arent over binded, and there is no air in the system it all should work.


http://www.bikebandit.com/1977-suzuki-gs750b/o/m6100#sch276406
 
Last edited:
If that compensating port is clogged it will do the same thing. It really is tiny and easy to block with chunks of rust or whatever.
 
Thanks for the replies and insight.

I will be going through it again later today.

Reply to Chuck Hahn

Installed spring/cups/piston exactly as you described. The piston fit nice and snug. I did hone the cylinder and clean it very well.

Reply to Evan132

I agree oem is the way to go. Anyways I just bought this bike and it had a spongy rear brake, you actually had to pump the rear brake to get to somewhat grab.

Upon initial mc removal/inspection I noticed that the circlip had fallen out of the grove (so all the internals fell out when I removed the brake rod) and the "washer looking deal" on the brake rod was bent...?...? What could cause that??
 
So I got to looking at the rear MC again, tore it apart inspected it and reinstalled it and "bingo" it worked perfect, (It was installed correctly originally) tested it and all seemed fine. Went for a ride around
the block and I was happy to have a rear brake.

As I pulled it back in the garage I heard the rear brake making noise, like it was grabbing a little, checked it, seemed fine. Later I was pressing the brake lever and looking at it and the damn thing locked up again. (Not bleeding back)

So i decided to order a new brake rod since mine was bent up for some unknown reason (after straightening it up) "washer looking deal that the circlip holds/retains" and a new stainless steel rear brake line just because I did not know how old the rear brake line was, the outside of the brake line seemed fine on inspection. Trying to narrow this down. If this does not work then all I can assume is that the piston is binding somehow in the mc.

The piston fits snug and makes vacuum/pressure and does not leak. I did not notice any wear marks inside the MC.

I hope I get this straightened out soon. grrrrrrr
 
What is the purpose of the outlined port in the mc (see attached image) It does not seem to go anywhere. Is it supposed too?

Well, The brakes are better, but now they drag.

I have been looking around on the forums for an answer and some people say there are 2 ports on the side of the piston cylinder, I only have one. So now I am confused.

I am starting to get frustrated, I have had the caliper and MC apart countless times

Here's what I have done.

1. Replaced the old rubber brake line with a stainless steel line
2. Rebuilt the rear caliper with factory parts & Cleaned (tested the rear caliper pistons with compressed air, they move in/out fine)
3. Adjusted the push-rod as per the Clymer manual.
4. Polished the piston cylinder to make it smooth (measured inside diameter, still within service limit)

The piston cup set does not block the compensator port when the push-rod is relaxed, the port is not clogged with debris.

The only other thing I can think of is trying a factory piston/cup set.

Unless I am missing something.

Thanks.
 
Anyone???

This does not make sense to me, as the seal (plunger above piston) stays above the bottom port in the mc, I can see the top end of the piston through the hole when relaxed.

I tried sticking a small wire (bread-tie)in the top port to see if there was a hole, no luck..

How is the fluid supposed to bleed back then?

I'm gonna look at it with a jewlers eye piece and see if I can get closer to fixing this problem.

Thanks
 
You should be able to shine a light through that hole and see it in the chamber. However, I had a similar problem a couple of years ago where, for whatever reason, the factory MC's return port wasn't drilled out. I had a shop professionally do it as the bit is so small I couldn't find it locally. It worked but I decided to upgrade to Salty Monk's twinpot setup and like it.

Check around town and see if a machine shop can fix it for you.
 
I would have replaced the seals in the caliper if I were that far. Not saying its your problem but relying on the original seals is never a good idea.
 
Halz426, were you able to fix this problem? I'm having the same problem and was wondering if you were able to try anything else?
 
Yea, I fixed it, the mc port was so clogged it looked like there was no hole.

The fix was simple. Took a piece of stiff wire out of a wire wheel or you could use a wire grill bristle and just work the hole.

Compressed air would not blow it out and all the other wires I tried were too soft and bent. Hope this helps.
 
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