RPM/Brake Correlation Question

[Chaldaean]

Driving home from work today, I accidentally tapped the brake pedal with my left foot while cruising at highway speed (right foot on the accelerator) and noticed that the RPMs increased by at least 50 RPM.

Is this RPM increase caused by the extra load required to replenish the brake booster? If so, is this normal or indicative of a leak?

 

[SiRaDuDe]

When pressing on the brake.

The Torque converter will unlock, and a rise in RPM is normal.

 

[Chaldaean]

Aha! Thanks, SiRaDuDe! So instead of the 1:1 ratio when locked, the input spins slightly faster than the output when unlocked. When the converter is unlocked, useful energy is lost due to friction etc.

Two More Questions:

Can this RPM increase be used to calculate the decrease in efficiency?

How does the torque converter on an automatic 80 Series lock? Is it a friction surface like a clutch in a manual drive vehicle or some other mechanism?