KZ 750 (4) brake issue

I picked up a 1981 Kawasaki KZ 750E (4 cylinder) a couple a months ago
and have been plagued with front brake issues. It has twin discs up
front. Here's the story:

After having the bike on it's side for a weekend to replace the starter
clutch the front brakes started feeling squishy. I bled the brakes for
2 hours and it didn't feel better.

I bought a vacuum brake bleeder and bled the brakes using that, but no
luck.

I rebuilt the master cylinder (it looked pretty clean) and re-bled. No
luck.

I re-built both front calipers. It was pretty dirty and one of the
pistons was probably 'stuck'. It took 4 times the compressed air
pressure to get the right piston out. I cleaned everything with brake
fluid and replaced all the rubber seals. I put it all back together
and bled the brakes again. I tried installing brake bleeder nipples,
but the ones I ordered didn't fit. So I put the stock ones back on.

The brakes are still squishy. My brother had the idea of using clamps
to squeeze the pistons slightly and see if that made a difference in
how the brakes feel. After clamping both pistons they felt pretty good
(not perfect). Is it possible the piston doesn't have enough travel to
get good pressure on the back of the pad.

My question is this. Is it safe to use shims between the piston and
the pad? Is there anything else anyone can think of to help fix this?


It's pretty frustrating.

Thanks for any advice.

-mauserdude

 

wrote in @g49g2000cwa.googlegroups.com:

Replace the brake lines?

 

Here's a logical strategy for bleeding brakes and getting all the air
out without buying some aftermarket vacuum pump...

Idiots? What were the original equipment motorcycle brake system
engineers thinking of, when they put the air bleeder screws down there
on the calipers? Were they that fucking STUPID?

Answer is: yes, they were that dumb. If you want to bleed air out of an
air over fluid system, you bleed the air out of the HIGHEST point in
the system, not a lower point!

Any time I have to bleed brakes, I don't mess with those stupid leaky
bleeder screws. They are messy and they leak air right back into the
system you're trying to get it out of. One of those silly vacuum pumps
can suck air around the bleeder screws, too, as can aquarium air line
tubing commonly used to bleed brakes by submerging one end in a jar of
fluid. Messy, messy, messy!

My method: I unbolt the brake calipers and I put a piece of 1/4 inch
steel plate between the brake pads. I let the calipers hang straight
down so the banjo fitting on the caliper is the highest point down
there. I loosen the master cyclinder bolts and arrange the handlebars
in a position where the fluid reservoir is HIGHER than its banjo bolt.


I arrange all the hoses so there are no high points for air to be
trapped in. When I pump the brake lever the air MUST go up into the
master cylinder. My brakes remain nice and firm after this careful
bleeding.

Aftermarket genius: Poking around on the internet, I found an
aftermarket brake system company (they make braided steel brake
hoses), and those geniuses came up with an air bleed banjo bolt that
fits on the master cylinder and sells for about $15.

All the owner has to remember is to cover everything up when bleeding
the upper banjo so he doesn't get nasty brake fluid on his paint. That
stuff removes clear coat PDQ...

You don't need to shim the pistons. The pistons are self-adjusting. The
square-section o-ring that seals the brake fluid into the caliper acts
as a rubber ratchet. It lets the pistons slide out as necessary, then
holds them in position. As the disk rotors wobble a tiny bit
side-to-side from normal run out and a little bearing wear, this knocks
the pads back a tiny amount, but the rubber o-ring can handle that...

If you have a whole lot of run out, say one of the disks wobbles
side-to-side more than 0.012 inches, you might notice that the front
brakes were very spongy when you first applied them, but you'd probably
also notice that the front brake lever pulsed in your hand...

 

snip

You had the bike on it's side? I hope this doesn't mean you actually had the
bike on it's side.

 

An alternative for the vacuum brake bleeder:
if your vacuum bleeder can also supply positive pressure (my MightyVac
can), you can _push_ the fluid in from the low point of the system. Of
course, this works best when the system is empty to begin with. You'll
also need to seal the threads on your bleeder screws. I used a couple of
turns of plumber's pipe tape. Pull the caliper off it's mount so that
you can hold the bleeder screw at the bottom. Attach everything, open
the screw and pump fluid in slowly until it bubbles out of the bottom of
the reservoir. You'll probably still need to do a bit of pumping to
clear bubbles from the MC, but you also probably won't have air in the
lines any lower.

I used this method on my clutch ('94 VFR, swapped out OEM lines for
braided SS) and got a solid clutch lever in one shot, no re-bleeding
necessary. Guys on the VFR mailing list I was on often complained about
having to re-bleed the clutch multiple times. I think that pumping fluid
into the system with the bleeder screw at the low point was a factor.

As someone else pointed out, some sponginess can come from old brake
lines. If yours are original to the bike, new lines should move (close?)
to the top of your list.

--
Later,
John



'indiana' is a 'nolnn' and 'hoosier' is a 'solkk'. Indiana doesn't solkk.