Rear brake pistons sticking?

[wirehairs]

I know... I thought I was done with my whole brake job... but no!  Ugh.  My rear pads seem to be clamping down on the disks enough though I thought I was done getting all the air out of the new Galfer SS lines.  But now here's the rub: if I bleed a little more fluid through them, the pads let up!  But then, 1/2 hour later, they will start grabbing again.  How can that be??  I'm worried the ABS system of this bike is doing something funny.  Now, they aren't grabbing so hard that I can't turn the rear wheel with my foot with some force, but you certainly wouldn't want to drive around with your brakes like that!

I imagine most of you will think I've got dirt in the system, but geez, I cleaned them out well, and replaced ALL the seals and rubber pieces.  Yes, if some genius here doesn't think of something else, I will drain it all again and take everything apart again, clean it, and try again.  I just want to make sure it has really come to that.   

[aviationfred]

I have never owned an ABS equipped FJ.  So pardon me if I seem ignorant.

Are you sure that it is not an ABS problem? A bad module, or controller?

http://www.ebay.com/itm/ABS-CONTROL-UNIT-ELECTRONIC-CONTROL-FJ1200A-ABS-92-93-1992-1993-YAMAHA-/131437075895?hash=item1e9a429db7:g:upAAAOSwBahU6rfw&vxp=mtr

http://www.ebay.com/itm/Y-93-YAMAHA-FJ1200-FJ-1200-ABS-CONTROL-UNIT-MODULE-OEM-/401071783714?hash=item5d61bdbf22:g:9V4AAOSwUuFWwMPp&vxp=mtr

Fred

[the fan]

Based on experience with my YZF (same caliper) I suspect that you have one of two likely issues.

1. Old hardened seals. The rectangular shape of the caliper seal acts as a spring to retract the pads back slightly from the rotor. When these get hard due to age or heat they will no longer do so.

2. Dirty or corroded caliper/piston. Crud buildup can prevent the pistons from retracting. This is fairly common on race bikes. I generally remove the pads and hardware from the caliper, extend the pistons and scrub it out with a strong degreaser. I generally use undiluted simple green and a tooth brush but common dish detergent will work. Do not use brake cleaner (DAMHIK). If corrosion is visible on the caliper or piston You likely need to rebuild or replace it.

[FJ_Hooligan]

Try pushing the pistons back a bit then reapply the pedal. 

This might "reset" the seals so they work like they're supposed to.

[Pat Conlon]

There is also a relief port in the master cylinder that can get clogged preventing the line pressure from relaxing when you step off the brake pedal.

[wirehairs]

These are all good comments.  Since I put in new caliper seals, I don't think the problem lies there.  But yeah, I guess I'm going to need to take things apart again and look closer for corrosion on the pistons.  This bike is very clean, so I didn't expect there to be problems, so maybe I didn't look close enough or something.  But before it comes to that, though, I think I will stick a flathead screwdriver in there and pry them back, just so see...

It's unfortunate how difficult it is to access that rear master cylinder!  It's so much easier on newer bikes. 

[ribbert]

To determine whether it's the caliper or the master that's keeping pressure on the pads, crack the bleeder when they are stuck on.
If it's the master it will immediately relieve the pressure, if not, you have eliminated it as the cause and can concentrate on the caliper (and vice versa)

Noel

[JPaganel]

There is also a relief port in the master cylinder that can get clogged preventing the line pressure from relaxing when you step off the brake pedal.

This  port (the spooge hole) has been the cause of the problem  on every bike with a sticky caliper I have ever worked on.

Clean your master cylinder, poke through the hole with a broom bristle. Replace fluid. I bet it will go away.

[ribbert]

This  port (the spooge hole) has been the cause of the problem  on every bike with a sticky caliper I have ever worked on.

Yep, as usual there are exceptions to every rule but sticking calipers are spoken of much more than they are encountered. IMO calipers left alone and in regular use give next to no trouble.
My experience also is that these problems are usually at the m/c, the previously mentioned test will identify that.

Noel

[wirehairs]

You guys were right; wish I had read the posts this morning before I yanked my calipers off!  Yes, those look fine, and the pistons look fine.  And indeed, by cracking the nipples, the pressure on the pads was relieved.

So I'll go and yank the MC off now.  It's no fun taking that one off.  Stay tuned to what I find in there.

[wirehairs]

By the way, the K&L rebuild kit has a different shaped and sized piston.  See below.  It is the same diameter, but is longer.  I hope that the difference in length is not inhibiting the flow of brake fluid? 

[racerrad8]

That is because the K&L kit is not made for an ABS bike they are different.

Here is the kit you will need.

92-93 ABS Rear Master Cylinder Rebuild Kit


Robert

[Charlie-brm]

Do FJ caliper seal rings have a tapered profile like this? (image from an XS750). If so, it's functional and relevant to the issue. If not, nothing to see here, move along.

[CutterBill]

ALL disc brake caliper seals have that taper. 

[wirehairs]

I thought the seals for the rear pistons were square, but now I'll need to look at them again when I get home.  I certainly don't remember my manuals making a note to make sure they were inserted in a certain direction, due to a taper.