I grabbed this from Cope's old website which might help:

On 727s & 904s with Trans Brake or reverse pattern valve bodies, CRT recommends that you relocate the vent to the rear of the case. The stock vent is in the pump cover, fluid will leak out of the stock vent when using a trans brake. Drill the stock vent hole with a 11/23 drill bit, then tap the hole using a 1/8 pipe tap. Install 1/8 pipe plug. drill hole at the 11 O clock piston in the rear of case as shown in picture above and install new vent.

Also block off the feed hole in the park gear, or you will have fluid spitting out the vent. See pic below