Disc Brake

The return hole at the bottom of the master cylinder is likely blocked with waxy crud from the old fluid . Disassemble the master cylinder and thoroughly clean it and clear the return hole with soft brass wire .

When you say "replaced discs and seals" guessing you were referring to the brake pads and caliper seals in your front brake. As you say sounds like pistons not releasing after brake lever released and as Richard suggests first thing to try is thoroughly clean master cylinder reservoir paying special attention to the small hole being clear. If problem persists I would suspect a problem with the new seals fitted to the caliper not allowing pistons to release after brake lever released , or clogged up brake pipe , especially if it's the original on the bike , not allowing free flow of fluid. Maybe a combination of both.

I had same problem after refurbishing original front brake on my MK1a 850 with new pistons and seals plus new brake pipe. Pads would brake nicely on the disc when lever applied but would only partially release causing drag and much heating up of disc. Suspected poor after market pistons or seals or combination of both. Finally gave up on the original set up and fitted a Norvil upgrade kit using the Lockheed Racing caliper and new disc which worked perfectly and still does several years on. 

.. that rather than HOT5 you mean DOT5. It is silicon, not glycol based and there are some doubts about its compatibility with DOT4 (which is still available and perfectly adequate).

I use(d) DOT5.1 which has reverted to the glycol base but has the temperature resistance of DOT5.

Dot 5 does not absorb water so any water that gets in collects at the lowest point as a blob of water, any steel in contact will rust.

Sounds like the caliper pistons are binding and pads not retracting - they should pull back slightly from the disc when the brake is released under the control of the caliper seals. You can try taking the wheel out and pushing the pads back. You should be able to feel whether they are moving freely. Pads should be "rattle" loose in the caliper body. Replacement pads can be oversize and very tight in the caliper body, file them down in this case. Corrosion is the most likely cause of the pistons not moving freely.. A brown residue in the master cylinder is almost certainly rust from the master cylinder piston so you have left the fluid too long without a change. You need to strip and rebuild the brakes.

DOT 5 fluid is incompatible with other brake fluids and really only appropriate for racing. If you have simply replaced the original fluid without stripping and cleaning the system beforehand you need to strip and clean now.

There is no good reason for using anything other than DOT 4 (the standard stuff) brake fluid. While DOT 5.1 might be useful in race conditions, it is less tolerant of water and must be changed more frequently.  DOT 4 is superior for everyday use; recommended fluid change interval is 2 years but you can stretch it to 5 years.

As a final comment to Stan's post which pretty much covers everything , agree piston corrosion is most likely cause and when you strip and rebuild caliper there are stainless steel pistons available. Original caliper pistons used back in the day were chrome plated mild steel and invariably in time the chrome would flake off the last few mm's of piston exposed to the elements with corrosion setting in.

I did try stainless caliper pistons on my original brake set up when refurbishing but after repeated stripping and trying again never got the pistons to release properly , although doesn't mean you won't have better luck. Binned the original caliper / disc and fitted a Norvil kit in conjunction with the original master cylinder refurbished - produced an excellent front brake on my MK1a 850.

My question:- If the brake worked ok before the work was done.  What exactly did get changed?  

I have found that a lot of the replacement stainless caliper pistons will not work correctly because they have the wrong surface finish, and can have a chamfer instead of a radius, thus damaging the caliper seal on fitting.   

 Original brake lines (rubber and steel bundy tube) degrade and can get blocked and/or restrict the flow of brake fluid. 

Some of the new master cylinders can allow too much lever travel before they start to move the caliper piston. The sleeve kits that are available for these master cylinders are superior in this respect as they are adjustable.

The system was designed for Dot 3 fluid, Dot 4 is perfectly ok.  Change fluid every couple of years.  

Fluid squirting from the hole in the master cylinder shows it is working ok and is not restricting the fluid returning to the reservoir. problem elsewhere.

Remove the caliper when bleeding the system and position the bleed screw at the top to allow the trapped air to easily bleed out. Use a Gunsons Easibleed or similar, makes it a bleeding doddle!

Remember to pack the caliper with a dummy brake disc when bleeding the brake this way, otherwise you will pop the pistons from the caliper bore.

regards

Peter

Be aware that using fluid other DOT 4 can cause the seals to swell a third larger, and that will jam the pistons, if anyone wants photos, I have them, it is scary. 

If you are using a 13mm conversion then you need to fill the system correctly, a doddle on a pre-MK3 and a pain on a MK3, But even on the MK3 you should be able to get a working brake in less than 15 minutes. It is due to the fill and bleed drillings in the caliper and the way they are orientated when fitted on the left and right hand sides.   

Evening Russel

Have you changed anything else? See my previous comments regarding bleeding the system, Ashley makes the point of the mk 3 being awkward to bleed. To make bleeding the mk3 brake easy, remove the caliper from the fork leg. This entails unbolting the mudguard brace and pipe bracket and flexing the brace inwards to free the caliper and pipe complete. This allows you to position the bleed screw uppermost.

If you are 100% positive that ALL the air is bled out and the lever still gives the impression that isn't the case, it will most likely be the caliper seals are holding the pistons away from the pads. See my previous comments about piston surface finish.