Maybe you have done this already, but I draw file the top of the m/c, try to get a new gasket (older gaskets tend to get harder) and increase the pressure from the bale.
There was another thread I replied to about a week ago that claimed same gasket malady. I suggested a perfectly smooth and level housing surface can be achieved by removing unit and dressing it using med - fine emery, or similar. Invert unit on flat (glass, plexi) surface, similar as when restoring aluminum t-stat housings. Rinse out insides when done - ie: inverted and brake-clean. Black gasket/seal should be soft enough to dig a fingernail into. Extra step would be to use some type of sticky sealer, just a teasing hint, on metal housing perimeter. It only takes a bit and no chance of contaminating the fluid. I have also used vaseline with no concerns. If you attempt to dress contact surface while unit is installed you risk having residue enter the system. In these situations, draw out most of the fluid and stuff some paper towel into the chambers. Then, vacuum out when done. I've had many new or reman units that had rough surfaces - right out of the box. :)
