MG Car Repair: 72 MGB rear end locked up, rear brakes, wheel cylinders

Question
Hi
 My 72 had sat for about a year with cooling problems, only bring rolled in
and out of the garage for this or that.  I just finished putting in  new water
pump,thermostat etc. and fired it up.  To my delight she started on the first
try and souned pretty good.  I drove it down the street carefully to check it
out got about a mile and the rear end seem to freeze up.  The brake fluid was
low and I had topped it off before I drove it.  Do you think the brakes have
jamed up or is there something else I should be looking at???  I was fortunate
to get off the road I felt kind of a drag getting worse fast as I pulled over and
then it froze, I could not push it the rear end would not turn.  
Any help would be great!! Thanks

Answer
Hi Kirsten,
It could be either the differential or the brakes so you need to jack the rear up and check.

Most likely it is the brakes. If it is still locked up after it cools off just open one of the bleeder valves on the rear brakes, if the wheels free up it is the brakes. If it is the differential you will be able to move a wheel just back and forth a little.

If it was the brakes pump the brake pedal up hard and check the brakes again to see if they have gotten hard to rotate again. If so open the brake lines at the master cylinder to relieve the pressure. If they free up, then the master cylinder is at fault. If they stay tight loosen a bleeder valve to confirm it loosens the wheel again. If that is so then you either have a bent or pinched brake line or a swelled up bad rear flex line. If they are still locked up and you have no small amount of free play in the wheel you could have locked up piston or pistons in the rear wheel cylinders.
Let me know.
Howard