Help- No brakes on '86 5kt when hot

[jwms@hpe.infi.net]

My '86 5kt loses the brakes when the car gets very hot from driving in
heavy traffic on a hot day.  The pedal goes to the floor but no brake
warning lights come on.  The brakes come back with a hard pedal if the
car is parked for 20 or 30 minutes.

Replacing the fluid with Castrol LTM Dot 4 did not solve the problem.

I checked the archives and found a similar problem on an 87 5ktq, but
could not find any indication as to what finally solved the problem.

I am thinking master cylinder but thought I would ask the list before I
start replacing things.

Thanks in advance,
Jim