First, we should call it a combination valve as proper terminology. The currently off shore produced MBM Brake PV2 disc drum combination valve that Cinnabar and others install (with proportioning!) has been found to have a very high multiple mode fail rate. They have visibly smaller fluid ports as well. The NOS GM brass valve number was sourced by Bob Stone and has had zero known defects over more that 10 years. The rear section rubber can easily be removed by end user and the “proportioning” function defeated, while retaining the hold off, safety shuttle valve and light switching (grounding) functions. I would talk to Bob or if you go on line to CPP (classicperform.com) there is a diagram with notes on removing the ‘rubber piston’. There is a “drought” of good options out there as GM Restoration does not license brake related parts, so no incentive to get them made by quality shops as reproductions. Also remember GM phased out steel valves (like our original) due to corrosion. Some 80’s GM cars were still delivered with steel combination valves, but by say 1990 all service parts were brass. Most GM valves are now depleted.