firmness of brake pedal?

[671M900]

I just finished bleeding front and rear brake sytems and I have a question. The fronts bled pretty well, they firm up after the free play point and squeeze and stops the bike pretty well.

The rear took a mightyvac to get fluid in and I did the same old pump till you get pressure, open the bleeder, close the bleeder, repeat. Pedal got firm. Really firm.

took her out for a ride and rear caliper seized after 30 minutes of riding, nearly lock the rear brake. I added some more freeplay, as I thought what I had set it at was too little, something like a millimeter. A recent thread had the same problem, but I was wondering if there were any other reasons? Pedal will go through full range of travel when bleeding, but once the bleeder is closed and one or two pumps, and then the pedal firms up.

How firm should it be? Once it passes the free play point, I can only depress the pedal something like 1/4th of an inch and then it's real tight. Grabs the caliper though, is this normal?

[ducpainter]

Firm is good.

Lock up is bad.

[671M900]

That's what I thought. I forgot to mention that after 5 minutes of resting after I found the rear had locked, it was perfectly fine to roll up my driveway into the side garage.

[ducpainter]

You should be fine if you added free play.

[Howie]

You need closer to 2 mm. of free play, minimun 1.5 mm.  Free play can be tricky to feel sometimes.  A way to double check if you have enough free play is to see if fluid is returning to the reservoir when you apply the brakes.  If not, back off the adjustment until fluid returns, then add free play.  If the fluid does not return and you have sufficient free play suspect a clogged port.  If the master is functioning properly and you still have a locking problem, make sure the caliper pistons return properly.

[671M900]

You need closer to 2 mm. of free play, minimun 1.5 mm.  Free play can be tricky to feel sometimes.  A way to double check if you have enough free play is to see if fluid is returning to the reservoir when you apply the brakes.  If not, back off the adjustment until fluid returns, then add free play.  If the fluid does not return and you have sufficient free play suspect a clogged port.  If the master is functioning properly and you still have a locking problem, make sure the caliper pistons return properly.

2 questions. How many ports are there? I've confirmed through pressure bleeding that fluid passes through at least one port. However, I don't know how many ports there are. I've even disassembled the cylinder to look at the piston and it had some crud on the spring, but I cleaned it off.

Second, how does one ensure the pistons are returning properly? I mean, when it firms up it squeezes the caliper real tight, when I let go, it's free.

It's very very firm, like when the brake booster goes out on a car, which I think it's supposed to be? Like it hits a brick wall so to speak.

[671M900]

Here's a video. Shows the freeplay (put in a little extra) and then the short lever travel afterwards. Fluid reservoir doesn't seem to change at all, except for when bleeding.



click to play

[Howie]

You should not see a change in volume in the reservoir, but movement of fluid.  There are two ports.  The names have changed over the years and even from text to text, but I will refer to them as compensating and replenishing.  The compensating port is in front of the piston seal     (high pressure side) and should be open when the brake is not applied.  This port allows fluid into the reservoir when filling and bleeding.  It also allows for the change in volume that occurs with temperature change and brake pad wear.  The replenishing port  is behind the piston seal and should be open when the brakes are not applied.  This port allows fluid to enter the low pressure side when the brake is applied and back into the reservoir when released.  What you are looking for is fluid movement in the reservoir not a change in volume.  You may need to remove the cap to see this.  What should see is a stream of fluid.  This i happens as the piston seal blocks off the compensating port.  Careful, it could squirt on your bike!

If you can physically see the caliper pistons retract, all is probably good there, but you might want to see how freely they move by carefully pushing them back in the bore.  If your bike has a rubber brake hose there is another possibility, though probably as rare as being struck by lightening.  The hose could have an internal tear that is working as a check valve, allowing fluid into the caliper but restricting fluid on the way back out.  Like I said, very rare.

[671M900]

Real quick before I went to work this morning, I put my hands on the sides of the pads and depressed te lever and released it. I felt the pads retract and release the rotor, but i didn't get a chance to check te actual reservoir. Will do so after work and before thanksgiving.

Thanks guys! And happy thanks giving DMF!!

[671M900]

Removed the rear master and disassembled it to check. Rear port has air coming out when I use the blow gun and feed a few psi through it. Front, I can't really tell. How would one clean the ports?

[ducpainter]

If you have the rubber parts out use brakleen

[DG]

had a SS lock up the rear and slide me into a curb! couldn't roll the bike for a half hour or so. totally "dished" and "blued" the rear disc. found out that the rear brake lever was not adjusted correctly (free play) and the master cylinder port was being partialy covered by the oring and after about 10 miles the fluid heats up and expands enough to close the pads...friction = more heat = more expansion = lock up & slide! fix was simply putting some extra travel into the master cylinder at the clevis. of course now I have a ton of toe travel! I live with it as I don't use the rear brake very much and kind of like the "wooden" feel anyway!