86 4x4 pick up brake problem

[scooterp7]

I have an 86 extended cab 4x4 pickup with 160k on it. While driving today I noticed a strange brake pedal feeling/problem. At certain speeds when I hit the brakes, the pedal is very stiff and and seems to stick hesitate before going down. It does not do this all the time. I can make it do it stopped by raising the idle speed and pushing the pedal down and it is not stiff then but still sticks. The connections to the booster look alright the vacum lines are itact and not leaking with soapy water applied. I dio not see anything under the dash that looked out of place . I do not have the FSM but I do have the Chilton manual. I did not see anything in there. I am at a loss to explain it. The verhicle still stops normally. Thanks in advance for any help/advice.
Scott

 

[Cube Dweller]

I know it's not much help, but I have the same truck and haven't seen this one before. I just did the rear brakes, pedal was mushy for a while, wheel cylinders were leaking.

Sounds like either the master or vacuum assist, which you already knew and were checking.

tony