Rear Disc brake conversion problem.

Converted my rear brakes to disc. Having an issue with brake pressure...

I have GM 6lug rotors and callipers fitted. Tried bleeding the system with the stock master cylinder and everything under the hood. No go. All the lines are new, no leaks, no air in the system at all. Takes about 4 pumps on the pedal to get all 4 brakes to activate. Figured it had to do with the OEM proportioning valve not sending enough fluid to the rear brakes... So i deleted the OEM one and replaced it with a manually adjustable one... still no go. Exactly the same outcome... 4 pumps then they all activate.

can anyone chime in? Im thinking maybe the OEM master cylinder/brake booster isn't sufficient enough to supply the needed push? Any aftermarket master cylinders that could work? or at least help?

let me know what you think.

based on its all 4 lines and with four pumps then they activate im going to assume its a pressure related issue, id start up front with the booster and go from there. not really sure however sounds like with enough pumps you build up pressure, if you continue to pump do you build more pressure or the same. in other words is it faster/easier to slow down with multiple additional presses or is it the same? perhaps a kinked line?

also if you go with a diffrent booster and master cylinder ebay has the chrome dual diaphram 7 and 8 inch ones for around a 100$

If you're having to pump them up, the master is too small, or there's still air in the system.

The mazda master cylinder is a 7/8 bore and booster work fine for rear disc. I know of 2 trucks running rear disc, one with a ford 8.8 and the other with a rx7 irs and they both stop fine.