I went for a ride the other day. All was fine until I stopped for a short while. When hopping back on the bike suddenly the front brake lever is free and loose for about the first half of its travel. Pulling the lever more will activate the brakes as normal. I have posted a YouTube video of this here...

https://youtu.be/nh3crL7qiWE

A friend reckons the caliber seals might of had it. Other research says it could be the master cylinder.
Just wondering if anybody knows whats causing my problem and what parts do I need to get?

How long has it been since you have flushed your brake system, or even inspected the condition of the pads and pistons?

Caliper seals could well be the problem. I would be inclined to remove the calipers and dismantle then clean pistons and bores with a scotch brite pad and thoroughly clean and reassemble using the original seals.  It may need new dust seals depending on how old they are.
I have had great success doing that.
The free play comes about when the seal wont let the piston slide freely and then retracts the piston too far after the pressure has been released.

X2 on Laitch & Rick's comments , I get this too when my pads are nearing end of life , the pistons have to move out a lot further this combined with old more rigid dust seals when the pressure is off the seals pull the pistons back as Rick outlined , the 1st bit of lever is used up just getting the pistons back out . Another thing you can try is to get your hands on an adjustable lever from a slightly later anti lock brake , this is what I have now

Good Luck

John Re

Melbourne Australia

The brake fluid is about one year old, changed by the previous owner just before I bought the bike. The pads are fine, but don’t know about the pistons because I’ve never had the calipers off. 
So it sounds like a caliper problem, not the master cylinder?
I’ll disassemble the calipers, put in new brake fluid, also might as well put in my new brake pads which I purchased some time ago as part of a large online order of parts from UK.
Maybe I should also just bite the bullet and put in new seals at the same time, the manuals say to always put in new seals when disassembling the calipers. I am guessing they are as old as the bike (1986), but I don’t really know anything about it’s service history. The rest of this bike is in pretty good nik, only 22,000 km on the odo.
Where would be the best place for an Australian to buy a seal kit, and I assume I need two kits to do both calipers at the same time. I see Munich Motorcycles in WA sells them for $48, or Motorworks in the UK has 2 different kits, both under £14 each.

my bet its the master cylinder ...take note of the 2 holes in the bottom of the fluid reservoir into the piston cylinder   easily seen from the top once the fluid is drained

ill bet the relief valve - bypass valve is blocked , not letting the piston return properly , that and a little weeping past the main seal would have it sitting in a bit more than normal

also have noticed overfilling the master cylinder reservoir will defiantly produce some similar issues ...so perhaps just  reduce the volume ?

had a member here with almost no braking available  from this overfill issue

michaelvass wrote:So it sounds like a caliper problem, not the master cylinder?
I’ll disassemble the calipers, put in new brake fluid, also might as well put in my new brake pads which I purchased some time ago as part of a large online order of parts from UK.

Nothing has been ruled out. What's needed here is analysis.

Charlie is pointing out one place to inspect; Rick and I are indicating other possibilities. I think it's too early to start replacing parts until it's necessary to do so. Charlie's suggestion is the place to start because it takes little disassembly to determine what's happening there. Just don't spill brake fluid on the tank and other painted surfaces because paint will be damaged.

If brake pad thicknesses are within specification, there is no need to replace them. Even caliper pistons and their outboard seal surfaces can be cleaned in place without disassembling the calipers and sometimes that's enough to return to safe riding.

Thanks for the replies. Will dismantle and clean the calipers and check out the master cylinder when I get back to the bike next week.

So what happened? Did you fix it?