Front Brake rebuild ?

My '98 WR400 front brake has gotten very spongey. It has been slowly getting worse for about a year. I have bled it repeatedly and many different ways. I already have a braided front brake line and the pads look good. I friend that used to be a motorcycle mechanic said I might need a master cylinder rebuild kit. Has anyone else had to rebuild their master cylinder?

I did this on a 99 YZ250. I was suprised about how simple the system is. Been a few years but as I remember it there is snap ring that has to be removed and a few pieces and thats it. Need to bleed it with a vacum pump and your ready to go. Good luck with it.

while working on your system, cleanliness is of major importance. You want ZERO foreign material introduced INCLUDING oil from your HANDS.

Either use Doctor's gloves and/or blast the master cyclinder clean w/ brake cleaner prior to your initial fill of the system.

[ September 12, 2002, 04:11 PM: Message edited by: Kevin in New Hampshire ]

Have you tried cycling the piston in and out several times, looking for binding? Many people rebuild perfectly good calipers only to find that the fluid had moisture built up in it, or the fluid has been heated so many times that it is toasted.

My front brake will get spongey from time to time, especially after the hard use of a couple of races. I generally have to completely flush my system which takes a complete pint to ensure a completly fresh fluid change. Then everything is great again.

Additionally make sure that the pin in the caliper is smooth and lubricated. After alot of usage the pin will get a groove in it causing the brake pad to hang and tilt a bit making the brake lever feel kind of spongey. Ty-Davis makes a steel pin that is really good at resisting scaring and grooving.

Also check your pads real good. Recently I replaced the rear master cylinders on both my YZ and my WR....Afterwards the YZ rear brakes were perfect but the WR would not lock up even with 200 lbs of pressure on the pedal....Come to find out the rear pads were glazed over from heavy braking and were not grabing the rotor....Some roughing with a med grit paper quickly resolved the problem.

Hope something here will help...

Bonzai :)

[ September 11, 2002, 06:04 AM: Message edited by: YAMAKAZE ]

I have had spongy issues too with everything in good working order.

The loop on the bottom of the line makes the system hard to bleed.

Removing the cliper and bleeding the system with the hose strait helps.

I also have had success with leaving a zip-ty holding the brake lever down. Just wrap a zip -ty around the grip and lever holding the brake on.

I read this in a chopper magazine and it relly helps when you want to get a 100% lever feel.

I tried the zip tie idea on my bike, and I left it overnight. I read a post about it on this board. But, it did not help. My caliper and pads seem to be in good working order. I used almost a half pint of fluid truing to bleed the system, so most of the fluid should be new. I just ordered a rebuild kit from Yamaha today. It was only $21. Hopefully it will help.

I cant see how this zip-ty thing is going to help? Please explain.


I think it is supposed to squeese the air to the top, so it won't be in the brake line.

Here is the thread I was talking about.


Originally posted by sabin:

I cant see how this zip-ty thing is going to help? Please explain.


With the lever pulled in the plunger in the master cylinder is "open," so air that is in the line can, by force of gravity, find its way up the line, into the master cylinder reservoir, and out of the brake system.

and if that doesn't work, it's time for a master cylinder rebuild. Which I will be doing tonight

I put in the rebuild kit, and rode the bike today. Wow, my front brake works good again. Although, it it was a pain to bleed the brakes again.

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