moto400ex

Driving today I hit the brakes normally to slow down and the rear tires locked up. I hit the brakes a few more times and noticed 1 clunk sound each time I hit the brake pedal. Any ideas to what this could be?

BuckB91XJ

If your hitting the brake pedal normally and the rears are locking before the fronts grab it could be a couple of things:

1.) Most likley it is air in the lines. Bleed them really good.
2.) Could be a bad proportioning valve.
3.) Could be a bad Master Cylinder.

98 JeepJeep

it may only be one side...you probally have a seized up wheel cylinder. only thing to do is remove both drums and look around.

moto400ex

I added brake fluid to the reservoir since it was just about empty and its not making the clunking sound anymore and its working a little better but the rear brakes still lock up a little.

BuckB91XJ

If the reservoir was almost empty then you definitely need to bleed your brakes!!!!!!!

jinx

would air in the lines make it so that your brakes would not work as well, not lock up? Just wondering because air compresses and that is why you get a soft or mushy pedal when they need bled. correct me if i'm wrong.

I would guess either proportioning valve or out of adjustment or wheel cylinder. just my .02

BuckB91XJ

Air in front lines would cause them to NOT work as well as the rears and would cause the rears to lock before enough pressure could be put on the pedal to make the fronts work.

JeepingDan

If the rear reservoir was almost empty it means you have a leak. I'd start by following the lines all the way to the rear drums. And then check the wheel cylinders. Yours shoes are probably soaked in brake fluid causing the lock up.

jinx

ah that makes sense. tunnel vision, I was only thinking of air in the back lines. thank you sir.

98 JeepJeep

you should never have to add fluid to the brakes.

Ultimate G

The last time my back bakes locked up on me, I found that the shoes had worn through and had to be replaced.

jeepin_for_fun

as a ase mechanic i would also check to see if the axle seals are leaking because 80w90 will cause them to lock up sooooo easy

moto400ex

Its all back to normal now. Added brake fluid checked all the lines and couldnt find anything

solderjunkie

agreed

agreed

Please figure out what is wrong with your brakes. You share the road with lots of folks who will appreciate it.

Pull the drums and have a peek. It doesn't cost anything and you can clean all the rust and brake dust out of there. I'll bet you find leaking wheel cylinder and contaminated swelled-up brake shoes.

SJ

Firestorm500

I don't agree. As brake pads wear, the caliper stays farther out of the bore. As it stays farther out, fluid has to be in the bore to take up the space. This will cause the fluid level in the master cylinder to naturally drop over time. Therefore, you will have to add a little from time to time to keep it topped off.

When you replace your pads and manually force the caliper to retract, the fluid will flow back into the master cylinder. Some may have to be suctioned out to keep it from overflowing.