spider brakes pressurising/binding

[farmer giles]

Hi, can anyone please help?
I have a 1972 spider and the last few times I have taken it out, after a couple miles the brake pedal seems to go hard and the brakes start binding, after a couple minutes it seems to ease and go back to normal.
Also if I don't drive it for a couple weeks the brake pedal goes hard in the same way and when pressure is applied it gives and works as normal.
I would guess this is a servo problem and pressure is building up when it shouldn't.
Any help or advise would be great, not really sure where to start looking!

Start looking at the rubber flexihose. When these break down inside they can give rise to the problems you describe. The servo wouldn't usually cause the brakes to bind - have you checked that the pedal is free linkage is free.

[farmer giles]

Just done some more road tests and the system seems to pressurize every 10mins/couple miles!
I have found that if I loosen the union on the small end of the servo nearest the bulkhead and relieve the pressure the brakes are back to normal!
I have just looked in the workshop manual and there is some mention of a pressure control valve and filter in the pipe from the intake manifold to the servo's, could it be that this relief valve is not working properly?
I will have to look into this, I have also heard of an option whereby you completely bypass the servos and fit a different type (Girling) master cylinder that is capable of powering the brakes with no servo assistance, does anyone have experience of this? I believe the racing boys prefer this method.
Just to add to the complication, a previous owner has already fitted a different master cylinder, a single system type. I would guess that the original dual system cylinder failed and they are quite expensive, so a single line system cylinder was fitted. (one pipe is capped off under the floor!).
If anyone has any ideas as to the cause of this brake system pressurizing problem I'd be very grateful to hear your ideas.