I have a 70 Dually CST 20 that was a wrecker in a former life. Converted to flat bed, cheaper to insure. retained the winch. It's a solid truck and good driver. I'm having trouble finding a master cylinder that will work with front discs and rear drums. I ordered the power brake booster, proportioning valve and master cylinder from Speedway and the master cylinder was bad. They sent a replacement that also seems to be bad. The front pushes fluid fine but hardly any dribbles from the rear. I had a Chevy dealer try to install the last master cylinder and they tried several from AutoZone, NAPA etc. and can't seem to find a master cylinder that will work. The dealer claimed the speedway master cylinder rear return port was too small and would not allow the fluid to refill fast enough. Could the plunger be too short causing the entire problem? This has me confused. Shouldn't be this big of a problem but I have noticed that others on the forum have had similar problems. Any help pointing me in the right direction would be much appreciated. I am at wits end.