Get yourself a new in-line valve from one of the list vendors for about
$10.  Replacing yours, which has aged and doesn't shut off the flow
completely, will be faster than bypassing the core.

Easy to replace, take out the spare tire and look up, you'll see it.
--
Stuart MacMillan
Seattle

'84 Vanagon Westfalia w/2.1
'65 MGB (Daily driver since 1969)
'74 MGB GT (Restoring sloooowly)

Personal mechanic for:
'70 MGB GT (Daughter's)

Assisting on Restoration (and spending OPM):
'72 MGB GT (Was daughter's, now son's)
'64 MGB (Son's)

Stripped and gone but their parts live on:
'68 MGB, '73 MGB, '67 MGB GT