Nige gave me a hard pedal a couple of times as did don recently.

Unlikely it's boiling. More like a sticking wheel cyl.

Are you running disks up front and drums rear? If it was drum all round did you remove the residual pressure bung? If not that causes the brakes not to release from memory

I checked the lines the closest point to exhaust is prob 50mm

Fluid was brand new Castrol suitable for dot 3 and 4...is that stuff alright?

Yeah its disks up front and drums in rear, only back left sticking though, thats the reason why i thought it could be the cylinder, but again it builds up over time so :S

I had a similar problem on my 1200. Basically the pedal became hard and my front right brake was grabbing and almost locking up.
I knew it couldn't be the callipers because they had been rebuilt, so I took the brake line off and cut it open. To my horror I found the rubber inside had degraded and almost blocked the line. Once replaced with a better line from the wreckers, brake bleed, no more problems!
If in doubt, chuck them out. Take the car to a brake place if still stumped.

My money is on the [new] master cylinder piston is not releasing back all the way and partly blocking the hole to the reservoir [the pressure will build and build]. Being a fixed length rod between the brake pedal and the MC, this can be an issue when replacing the master cyclinder. [This is all based on the assumption your car doesn't have vacuum assisted brakes]. There are shims availble that sit between the firewall and the MC to obtain the correct position.

cheers
benny

Thanks everyone for the input lots of info flowing!

ogyris-yeah mate if i can't figure it out i'm just going to take it to a brake place.....try and save my $$$ first!

Benny-i'm going to give that a go, could you get away with using washers?

Thanks

Someone on here said this once. If you can't run water down a brake hose, while its off the car, then you need to replace it.