Last Updated on Saturday, 23 October 2010 15:13 Written by Mik Tuesday, 01 June 2010 23:52

Bleeding Brakes and AVDS.

 

On a bike that is around 25 years old, it goes to say that the brake lines will also be very tired its difficult to get new lines for the GPZ these days and even if you do manage to get NOS, they are still old and with age comes hardness/brittle and general rubber breakdown.

I replaced my front lines with some coloured HEL lines, they are nice and tight and the colour looks good on the bike, this is the only part (that you can see) that isnt original on my bike.

The AVDS presents its own problems as they are linked to the bleeding of the brakes, I spend quite a lot of time getting this right and I have NOT blanked off the anti dive system.

Buy the best brake pads you can afford and fit them into the callipers and onto the machine.

First I use this trick and I  pull the lever all the way in, wrap a bungie cord around the lever and grip and let it sit like that overnight.

The constant pressure will generally force air bubbles to the top where they can escape through the master cylinder.

BE SURE that the tiny bleed hole in the floor of the reservoir is clear and clean. Any dirt/debris or old fluid gunk will block the port and they will NEVER bleed correctly.

As my bleed system I use some clear tubing that I bought from a auto shop pretty cheap, an old jam jar and drill a hole in the lid, I feed the tube through the hole in the lid and push it to the bottom of the jar, I fill the jar around a 3rd full, then push the other end of the tube onto the bike nipple and loosen the bleed nipple.

when you first start your bleeding it will take a long time to get any fluid down to the nipple outlet (possibly 10 mins or more of pumping the lever)

I then start  bleeding from the bottom nipple first and working my way up the various bleed nipples. This takes a long time but worth the effort

Once I complete one side of the bike, I then start the other side, its really only then you start to feel a good pressure at the brake lever.

I then do this all over again until I am happy that I cannot pull the lever back to the bar. If the lever feels spongy, then you still have air in the system and need to find out where the air bubbles are.

 

 

 

AVDS

 

Replacing fork seals is a frustrating job in itself, but what is more frustrating is finding that you then have to do it all over again in a few months time. When you get fork seal failure in just about every instance you will find the problem lies somewhere else and the blown seals are the symptom not the problem.

One area to look is the Anti dive units or AVDS, over the years these have served riders well, but when they start to fail they usually end up being blanked off, not because its not liked but because of the issue to tracking down parts and when you do find parts they are usually expensive running into hundreds of dollars. It is important that owners of the GPZ600R understand this system and with knowledge gained it will be easier to pin point other problems.

 

How do the Anti Dive units work?

 

The AVDS on the GPZ600R is designed to stop excessive compression on the front suspension under heavy braking, This will allow you to brake later into a corner or on the straight at speeds and they also give you greater control during emergency braking.

 

The AVDS system uses a variable control in the damping unit (albeit fairly limited) on each for leg. This control valve is housed in the silver fin at the bottom of the fork, at the bottom of the valve is a black adjuster disk which can be moved/adjusted to positions 1,2 and 3. It is important that both disks are set to the same number. Number 1 being the softest and number 3 being the hardest.

 

When you pull the brake pressure is diverted to the plunger valve, this plunger closes the control valve of each Anti Dive unit on each fork leg, the more that the braking pressure increases the more the plunger valve closes and restricts the flow of the fork oil, thus making the fork suspension stiffer and preventing excess travel.

When the pressure is released from the brake, then plunger returns back to its normal position (off). This in turn allows the forks to return to its normal resting position and thus allowed to respond to normal road conditions.

 

 

 

 

 

 

 

What goes wrong?

 

The most common and worst fault is that the unit/plunger seizes on, and this in turn makes the suspension permanently stiff (not a good feel) this can lead to poor handling, and/or fork seal failure.

Most of the cases is that after 20 years the return spring has just lost its bounce and no longer return efficiently.

 

 

The “O” rings can also fail, this then jams in the valve and also causes problems. The “O” rings can also cause leaking hydraulic/brake fluid.

 

Air in the braking system can also cause problems and the plunger is no longer depressed and thus can eventually seize up.

 

You can also end up with sludge and corrosion in the system.

 

The disk/adjuster can also fail (It actually brakes away from the pin inside) and it ends up just turning without actually making any adjustments. You would usually need to buy a used or new unit to

replace this item.

 

Maintaining our Anti Dive Units.

Make sure you always bleed all air from your brake system.

 

Bleed all of the air nipples, on the 1985 model there are 3, one on the Anti dive unit, one on the brake calliper and the other at the junction.

 

At any time that you strip the forks, replace/overhaul callipers get into the habit of also stripping down the Anti Dive unit, replace the “O” rings and spring and make sure the adjuster disk is working how it should. You can get a kit from GPZZONE (link on web-links) and they are not really expensive.

 

Try not to be tempted in fitting a blanking plate all this usually does is restrict any oil flow and you end up with a stiffer permanent suspension which makes for a uncomfortable ride. A special blanking plate need to be used as does other items such as having your damper rods drilled and re spring your forks, so best get the professionals to do this.

 

 

 

Disclaimer here.

I am not a motorcycle mechanic and have been restoring/fixing bikes for only 35 years, if you are not competent in your own work, I would suggest you seek professional help in anything that you need to do to your own brakes.  Brakes are the biggest safety feature of any motor vehicle and it cannot be stressed enough that ONLY DO THIS IS YOU HAVE EXPERIENCE. No liability accepted.