Front brake help please - piston not compressing

I need a little help please, kinda stumped.... I will try to describe everything

I recently purchased an 82 XJ650 maxim (single piston, single rotor). I rode it around the parking lot when I first bought it. The front brakes felt a little spongly. I couldn't lock the front tire up if I tried. I figured there was air in the line and a simple bleed would solve the problem. I didn't do that right away though. Since I was fully disassembling the bike after that first ride, I figured I would bleed the brakes and check everything out upon reassembly.

After I removed the front wheel for repaint, I fully compressed the piston caliper with a piston compressor just as you would on any car/truck. I did this to make putting the wheel/rotor back on a little easier. The brake pads looked ok, a good amount of material left. So I left them as they were. After I got the wheel back on, I checked the fluid level, bled the brakes, no air in the line, then went for a ride. Well.... no front brakes at all now!!! I had already pumped the brakes, just as you would if you compressed the piston all the way on any car/truck. Still, this did not solve the problem. I took the caliper off to take a closer look. I thought, maybe I had siezed up or damaged the piston when I compressed it. But this was not the case. With the caliper sitting in my hand and cycling the brake lever, I could slightly see the piston moving. But when I released the brake lever, the piston would move back to its origonal position, is this normal???? Maybe there is a check valve in the master cylinder that is not functioning???? Any ideas on this please. I am very familar with automotive braking systems, but is there something on motorcycles that I am missing?

Any suggestions would be great. Riding around with only rear brakes is no fun, lol.

 

Could be the master cylinder relief port is clogged, which is a common problem:

http://xjbikes.com/Forums/viewtopic/t=8 ... elief.html

 

That sounds like it could be it. Something clogged. Because the brake fluid is pretty nasty dirty. I'll read through that thread. It will tell me how to clean it out? Thanks for your help Len, I knew you would chime in!

 

I cleaned out the two ports, they were actually not clogged. Does anyone have any other suggestions? Thanks

 

I rebuilt my master cylinder today and have a question.

After rebuild, do you have to 'bench' bleed the master cylinder as you would on a car? All I did was fill the reservoir with fluid, cycled the brake lever a TON of times and I still didnt get any fluid/pressure to the brake line at the caliper. I disconnected the line at the caliper, and nothing... any suggestions??? I also noticed when cycling the brake lever, there was air bubbles coming through both orfices in the floor of the master cylinder reservoir.

 

If you have a "dry" system (meaning the calipers and or lines have been emptied of fluid), it takes FOREVER to fill them again. As in, "a good long time bordering on forever" if you try to manually pump them full of fluif. These master cylinders just don't move a bunch a fluid like on a car. If you have a vacuum bleeder, use that! If you don't have a vacuum bleeder, trust me, BUY ONE!

Then you can fill and bleed a fully empty system in about 3-5 minutes.

ALSO, check your bleeder screws! If they're clogged with junk, you'll never bleed your brakes. Happens more often than you think......

AND.....if you have installed "speed bleeders" on your caliper, you'll NEVER bleed an empty system. The air in the system will fully compress before the one-way valve/spring in the speed bleeder ever releases.

 

I started over with a dry MC on my Suzuki and it took hours for me to get them to bleed. But what ends up happening is when you pull on the lever the compensation port (the first very small one which make the initial lever pull softer so you don't lock up the front tire right away) lets out air and the remainder of the lever pull doesn't compress the air much. It's a bit counter to bleeding with fluid, but what I had to do was open the bleed valve first, then pull on the lever, close valve, and release lever. MAKE SURE THE MC PLUNGER COMES ALL THE WAY OUT TO THE LEVER PIVOT! If it does not, wiggle the lever against it until it finally comes out. Repeat the steps above until you start getting fluid to come out at the caliper. Then use the traditional apply pressure, open valve meathod to finish bleeding. If the fluid is old, do a couple MCs full of BRAND SPANKING NEW, NEVER OPENED brake fluid making sure to never suck air into the line by allowing the level to dip down far.

Yes, the design of the calipers uses square seals which deform in specially machined bores when the pistons extend. The result is that the seals pull the pad/piston(s) back away from the rotor to prevent brake dragging when the pressure is released. I used to work for the company that makes H-D brakes so I've gotten to play with them quite a bit and know what is on the inside of them.

 

Awesome, thanks guys. Sounds like I need to just get some fluid in the system before I go to bleeding. Because I did completely drain the lines also. So this would be considered a completely dry system right now. I will apply your suggestions and see what happens. I have the stock bleeder screw on it right now. I'm sure it's not clogged because I tried bleeding the brakes to solve the problem (before rebuilding the master cylinder) and I got fluid to come out of the end of the bleeder screw when I loosened it. Thanks guys!!!

 

I did the exact same rebuild this spring and added a rebuilt to the cylinder. It does take a very long time to get it working well manually. Just keep at it and after you think you have all the air out, tap the various "hard" components to try and jiggle any air that gets trapped in the corners. I had to bleed it a few times over a few days to get good brake response and may do it again tomorrow just to make sure.

 

Pvtschultz---very interesting. So that small port in the floor of the reservoir is to prevent the brakes from locking? I always thought that the purpose of it was to allow fluid to bleed back into the cylinder to prevent brake drag.........hmmmm.

I never realized that the square seals are what pull the piston back. I though a square seal merely had a larger contact area with the piston and thus provided better sealing (vs. an o-ring style seal).

 

I learned the same thing in college: Automobiles use a square cut seal for the caliper's piston to assist the piston to a retracted position. There is also a very slight amount of rotor runnout designed in to the machining to further assist this. Only about 0.005"

You're right, tapping the hard components lightly should help release air bubbles from the surface.

 

I was partially correct now that I have thought about it some. The small hole is to give the lever some travel during light braking and then restricts the flow during a quick grab of the lever during hard braking giving it a firm feel. It also allows that small amound of additional fluid to get in front of the plunger but usually the seals lay down some so fluid can move from the back to the front of them. I've been out of the business for a bit now, but that is what is coming back to mind. It really is a multi-functional hole though.

 

To assist in bleeding you can unhook the brake line from the caliper and submerse the end into the master resevour. Then pump away until you have no more bubbles. Re-attach the line and continue bleeding.

Sean Morley

 

Oh, no...

Cover your ears, Verna! It's about to get dirty!!

 

Crath:

You have to overhaul that Caliper. There are channels for the Caliper Seals.
Those channels will get an accumulation of brake dust, road dust, brake fluid and moisture that turns into a hard substance that causes the Caliper Piston to bind.

The net effect of the buildup is to render the circumference of the Seal within the Bore to be "Out of round" ... hense, the Piston's normal ability to travel within the Bore is difficult.

You need to remove the Caliper
Remove the Caliper Piston
Remove the Caliper Seals and scrape the Channels perfectly clean.
Fabricate a "Scraping Tool" to scrape the foreign substance out of the Channel down to bare aluminum.

I use a wire coat hanger and snip, flatten and bend the end of the wire to be a scraping tool.
You have to scrape the "Junk" out of those Channels and whatever accumulated in the rear of the caliper in the fluid space.

Once you have the Channels all scraped clean of the hard build-up, you can reinstall the Seals and place the Piston back in there and expect it to respond freely to the changes in hydraulic pressure.

Use the opportunity to clean and lube bushings and other sliding points with waterproof synthetic grease.
Then, when you get the Caliper rebuilt and remounted ... it will "Pump-up" and release like it should and you'jj have decent front brakes.

Two and a half hours ought to do it.
Less if you dont have to struggle getting the Piston out of the Caliper.

 

awesome, that's what I'll do this weekend chacal, thanks!! More things on the 'to do' list. That is a good things for me to do with this bike. The inside of the caliper probably looks just as bad as the inside of the master cylinder resivior.

 

Go buy a couple of Cans of spray "Disc Brake Kleen"

You're gonna need 'em!

 

you've cleaned it, you know it's clear, but where is the fluid. It takes fffff-forever, pump slow you will get there.
P.S. watch where that fluid goes, if it gets on your paint job, it will seriously ruin your day.

 

Yeah, put a clear hose over the the bleeder to direct it away from all painted surfaces. Plus this way you can watch for bubbles in a more controlled manner. DOT3 and DOT4 are quite corrosive.

 

Yea, I got some on the tank last week. I just repainted it last night because of it. Brake Fluid + paint = BAD