Originally Posted by SaM91
Yes, Coan built it new, rated to hold 1200 HP.
It is regular trans break shift pattern.
No, there are no return spring in the drum.
Yes. Bleed off hole contains a small ball in there. Acts like a valve.
there will need to be return springs on the piston in the drum. usually stiffer springs are used in trans brake application.
Bleed hole is added even if drum has the OEM factory check ball valve as you mention.
there are multiple reasons for direct clutch pack failure. Low line pressure, leaks at case and center support meeting, sealing rings leaking, shifter adjustment.
The frictions can also burn if they do not release quick enough during trans brake operation. This is where the stiffer return springs help , the bleed hole helps release the clutch pack.
Clutch pack clearance is important too. A 6 friction pack I use will be set up in the area of .070 total clearance. Different builders will have their own desired clearance setup
Coan may have an additional bleed hole built in that is not seen by a inexperienced performance transmission builder.