[ Nate Otto wrote in 190318 MMDigest:

> Top (inside) valve facings are neoprene, exhaust is cabretta. 

Valves should be covered in leather made from cows.  The valve has to
have a little play or wobble on the stem in order to seat.  Neoprene 
will not let the valve slip or slide into place to seat.  Cabretta is 
sheep skin and will leak.  

All valves have to seat against leather.  Leather with the rough side 
out seats best against wood, smooth leather seats best to brass valve 
seats.

Don Teach
Shreveport, Louisiana