LHS Front Brake Caliper 'Auto' Closes - What to Do?

[dcarver]

So then. Heavy maintenance on KrZy8 with almost 250k.

Today was install front suspension and brake pads. EzPz. Been there done that.

But Noooo..

The LHS front brake caliper, the non-linked side, the pistons are 'auto' closing. They will not stay in the 'spread' position.

So I went to the RHS, the rear-front linked side, and all is good. The pistons stay 'spread' to allow caliper installation onto disk.

The LHS though? The pistons are closing sufficiently to hold two fairly large screwdrivers between the pads. And they pistons move pretty darn quickly too.. way too fast to install onto the rotor.

Never seen this one before, any ideas?

 

[RossKean]

Brake reservoir over-filled?

Can you push the pads back and have them stay if you open the LHS bleeder?

Is the upper piston pair on the RHS behaving?

I wonder if you have a blockage in the hose to the left caliper?

Is the ABS working on the front - maybe something wrong with ABS pump but I think that would affect both sides?

 

[Redfish Hunter]

My first guess would be a bad brake hose restricting flow back into the master cylinder. I have seen this before on trucks. How old are those brake hoses?

 

[gregory]

Gunny for RFH

 

[philter]

The only thing my peabrain can figure is that the four pistons in the left caliper are loosey goosey and are being pushed back closed by the trapped air in your master cylinder, and the pistons in the right caliper are a little "sticky" and therefore aren't responding to the pressure.

 

[dcarver]

Brake reservoir over-filled?I'll check, but probably not. KrZy8 has been sitting lonely in the shed while Naomi gets all the love and attention.

Can you push the pads back and have them stay if you open the LHS bleeder?

Will try this later today.

Is the upper piston pair on the RHS behaving?

Yes

I wonder if you have a blockage in the hose to the left caliper?

Intriguing idea. But what could cause that good of a blockage in a contained system?

Is the ABS working on the front - maybe something wrong with ABS pump but I think that would affect both sides?

ABS only links to the RHS caliper which is working sat.

My first guess would be a bad brake hose restricting flow back into the master cylinder. I have seen this before on trucks. How old are those brake hoses?Well, they are OEM circa 2006.... so they owe me nothing.

The only thing my peabrain can figure is that the four pistons in the left caliper are loosey goosey and are being pushed back closed by the trapped air in your master cylinder, and the pistons in the right caliper are a little "sticky" and therefore aren't responding to the pressure. ​Very unscientific on my part, but RHS and LHS caliper require the same amount of force to spread the pads e.g. compress the pistons.

I'm slacking off today riding Wabs around the county. It's going to be 82F this afternoon, the leaves are starting to change and fall and the stupid deer are getting into even stupider rut season. Damn near hit one the other day. It was waaaay toooo close.

 

[gregory]

Beware Bambi and enjoy your ride!

My vote is still on a restriction not letting the LHS fluid back. See if it gets better the longer you leave it alone

 

[RossKean]

The rear ABS circuit links to the lower front right but (as far as I know) the separate front ABS circuit links to the upper RHS and both LHS?

I think I might start with bleeding a bunch of fluid through the brake lines and see if any junk comes out. Brake lines may be bad but these seem to last a very long time.

 

[dcarver]

2018-10-20 BrakeWoesFixed



With gratitude to FjrForum peeps and personal friends via email..

YEP! Problem solved. Wayh overfull master cylinder.

Records show I flushed the system about 4 months ago.

With skinny pads.

And I tend to put fluids at high level marks.

New 'fat' pads installed and, guess what? Fluid has to go somewhere.

When I lifted the MC cover.. brake fluid gushing out.

Mystery solved

Thanks again, Peeps!

 

[RossKean]

Thought that might have been a possibility. Beats the heck out of having to replace brake hoses! (Or a bad ABS block)

 

[Redfish Hunter]

Good Stuff. Since I never saw a master cylinder produce more fluid on its own I never considered that possibility.

 

[RossKean]

Replacing skinny worn pads with new ones will push back the pistons and push more fluid into the reservoir. Brake fluid volume can also increase with absorbed moisture.