Could be low pressure for sure. Remember the transbrake apply is a timing thing. When the solenoid moves the valve it sends low gear fluid pressure to reverse, so for a moment low pressure drops till reverse is applied, the key is have reverse apply and low "reapply" at the same time. So along with low pressure, a bad ring on the low servo, a band out of adjustment, and not enough clearence in reverse can all cause this problem. Check your pressure at the low servo cover and see what you have at idle.