Front brakes locking up

[PJSTED]

I am nearing the completion of the restoration of my 1979 MG Midget  and am working some of the bugs out.  One of my nemeses are the brakes.  I have installed a new master cylinder and rear wheel cylinders.  The front calipers have been rebuilt.  All the hoses have been replaced with new and all the lines have been removed and cleaned.  The warning valve has been cleaned and new o rings installed.  Dot 5 silicon brake fluid has been installed.

As I drive the car, initially, the brakes seem to work fine, no spongy feeling.  However, after a few miles the front caliper lock up.  I have checked for leaks and cannot find any.  I have bled the system a few times following the procedure in the Haynes manual.  This is a split system between the front and back and I bleed the right (front and back simultaneously) and then the left.  I do not notice any air bubbles when I finish the bleeding.

Does anyone have any idea what is causing this problem?   

[Art]

Hi Paul,
 
With all that you've done at the same time, it is easy to not be able to pinpoint any one item for this issue. 
 
I would tend to focus on the proportioning valve or even the rebuilt master cylinder.  If the pressure builds and the front brakes lock, it could be more that the fluid is not returning to the resevoir keeping the cylinders extended.  The things that would cause that are the return restrictors in the master cylinder (fluid does not return as quickly as it arrives at the wheels to help the driver better control the brakes) or the proportioning valve, which blocks-off one system in the event it senses a failure (front or back; left front/rightrear or vice versa, depending on how you car is plumbed). 
 
The other thing to check would be to make sure that the pistons in the front calipers are not too tight and binding as they get hot with use and the loosening as they cool.  This can happen especially if the pistons were replaced without honing or bushing the calipers.  From what you've said, I think if you find nothing else obvious, consider having the calipers professionally rebuilt is the next best step. 
 
You migt check with Apple Hydraulics in NY.  They specialze in classic car hydraulics, with attention to LBC's.
Their contact info is below:

Apple Hydraulics, 1610 Middle Road, Calverton, NY 11933
Voice: 800-882-7753, 631-369-9515, Fax: 631-369-9516
e-mail: info@applehydraulics.com | home page: www.applehydraulics.com

 
I hope that helps you.  Let me know how you make out.
 
Art