c4 PA transbrake trouble

I activated the t-brake reved to 3000 and everything was good, reved to 3800-4000 and it started to jerk and slip. I dropped back to idle, let the t-brake off. I then re-activated the t-bake and reved to 2500 and it was jerking and sliping.

If I hold the cars brake, then rev to 3000 with the t-brake then let the cars brake off, it will hold and wont jerk or slip, but if I dont push the cars brake fist it will jerk/slip.

Its a PA full manual reverse t-brake and this is the first time I've used it...or any t-brake for that matter.

Whats going on??????????
My stall should be 3800-4000

anybody?

A tranny brake requires a really good tranny if you are going to do some stall testing.
I think the C-4 brakes apply the forward, 1st gear band and the 2nd gear band to cause the lockup. I would take a good look at the bands & drum surfaces that the bands run on to make sure they will grip well.