rear master cylinder is pissin me off

Hi folks, I'm preparing to re-assemble my '78 kz1000a2 rear disc brake caliper. I'm struggling with how to best put the brake pucks back in to the caliper.

So I have only done one prior rebuild of a disc brake caliper (an early 1970s Honda). It was a struggle to push the puck back into the caliper.

What is the hot setup for this? For the Honda, I used saliva to lubricate both the rubber seal inside the puck's chamber area and saliva on the sides of the puck, figuring saliva would be harmless, while using a different lubricant would be a FAIL.

But still I had to use a block of wood and a hammer to pound the puck into the caliper. And this worked okay, the Honda's disc brake has worked fine ever since, but I don't like 'hammering' anything like that.

What's a better way? What lubricant, and what tools do the smart folks use to put the puck back inside?

its a little late but if you have stuck bolts ect. soak in vinager... you will be shocked at how fast that works

BadAssRX7 wrote:

its a little late but if you have stuck bolts ect. soak in vinager... you will be shocked at how fast that works

Thanks dude, I will try that next time, I've never tried it. Is it safe around rubber parts?

Use silicone greese next time! Used it on my first calliper rebuilds on my KZ1000 J3, just pushed the puck in with my fingers with that on.

This is an old topic so I'm not sure if anyone is monitoring this.  I am restoring a 'barn find' 1977 kz1000a and the last issue I am facing is the rear brake master cylinder and it seems to be exactly the same issue described here - I am unable to get fluid into the cylinder between the hose exit point towards the caliper and the master cylinder piston.  I can only see 2 holes in the master cylinder itself - one hole comes from the reservoir via a hole under the big hex bolt (it is drilled at an angle into the cylinder).  The other hole is the exit point to the brake hose to the caliper.  Now, I noticed that in the area beneath that big hex bolt there appears to be another 'hole' into the cylinder  - but this hole does not reach the cylinder.  It is not filled with 'gunk' it is just partially drilled.  This is in the right place for the relief hole mentioned in this topic.  As it stands, I am unable to draw fluid from the caliper or pump fluid into the cylinder from the brake hose as once the piston is installed in the master cylinder there is no way for air to enter or exit that area between the cylinder and brake hose exit point.  Fluid is only reaching the cylinder via the 'main' hole. I would assume this would all work if the relief hole existed but I am reluctant to just drill that partial hole through and potentially destroy the master cylinder.  The attached phot shows the 2 holes under the big hex bolt.  The one in the middle does not go through to the master cylinder (could be the relief hole?) and the one above and slightly to the left does go through and fills the cylinder with fluid.

Does anyone have any thoughts?

Ross

There are two holes in the bore. The pressure return hole is likely blocked

 

When I rebuild my rear brake system I needed to vaacum bleed before I could build up pressure with the master cylinder….

rshirrefs post=868749The other hole is the exit point to the brake hose to the caliper.  Now, I noticed that in the area beneath that big hex bolt there appears to be another 'hole' into the cylinder  - but this hole does not reach the cylinder.  It is not filled with 'gunk' it is just partially drilled.  This is in the right place for the relief hole mentioned in this topic. 

Mikaw is exactly right.  You need to open that small hole.  Many many folks have had issues with this hole being clogged.  It may seem like it wasn't drilled, but the small relief port is absolutely necessary for the system to work.

 

This is where a torch tip cleaning tool comes in handy.

Firstly, thank you to those kind people who provided feedback to my post and secondly apologies for being tardy in responding - I just wanted to get to a resolution to what has been a long saga.  The issue with my brake master cylinder was indeed the small relief hole being blocked.  The odd thing is that it looked like it had never been drilled through and try as I might I could not find any suitable implement small and strong enough that might unblock it.  In the end I decided to try my luck and drill it through at the risk of destroying the master cylinder.  I had some tungsten drill bits of between 0.1mm and 1 mm.  But with these I only succeeded in destroying the drill bits.  After drilling through the cast aluminium I could see that there was indeed something blocking the hole and it wasn't going to give up without a fight - it looked like it might have been another broken drill bit from someone possibly facing the same issue.  As a last resort I bought some cobalt tipped drill bits (0.5 mm was the smallest I could get) and the first attempt drilled right through.  Yay!  Was the hole going to be too big or too small - I had no idea.

So I assembled the master cylinder with the rebuild kit  and attempted to bleed the brake.  Absolutely no luck.  - couldn't even get any decent fluid pressure.  I tried forcing the fluid in from the caliper with a syringe (which worked) but I still couldn't get any fluid pressure.  Next stop was a vacuum bleed.  Same result.  I could suck the fluid through but couldn't get any pressure.

In the end I decided maybe the master cylinder was just worn out and I was contemplating getting it re-sleeved.  So I disassembled it and then I noticed that the front rubber cup was not properly positioned in the master cylinder when I removed the piston.  That would explain the lack of pressure issue.  So I carefully re-assembled it and this time I was successful in getting fluid pressure and finally bleeding the air out of the system.  In the process the rubber cup may have deformed somewhat as there are signs of a small fluid leek (yet to be confirmed) - but at least I know now that master cylinder itself appears to be operational.  And I now have a functioning rear brake.

I hope this post might be useful to some people.

Ross