Lee,

I don't know for sure, but I think that your older bus expects the gas cap 
to include a vent.

I am reply quickly to this because I think that you could be creating 
yourself quite a problem.  I believe what you heard was not pressure, but 
vacuum.  There is a very real danger of collapsing your gas tank.  You 
wouldn't think that a little fuel pump could do that, but trust me, it can 
and will.  (I hope that that hasn't happened already!)

Later models have a fuel vapor recovery system that requires the gas cap to 
seal, not vent.

The solution is to drill a small hole in the gas cap.

I hope that this note gets to you in time,
Mike

Michael A. Radtke  -  Z74               Voice: 602-862-4897
Bull                                      FAX:         4853
13430 N. Black Canyon Hwy.
Phoenix, AZ  85029                      Email: m.radtke@bull.com