Having a lot of difficult bleeding the brakes

[Chambers]

Hi all, I installed new stock replacement calipers in the rear and I started to bleed the system with my buddy but the pedal would get a decent amount of pressure with the engine off, but when it was turned on there was no pressure, it took about seven to ten pumps to get just the smallest amount of pressure with the vehicle running, I made sure the rear bleeder screws were pumping out all brake fluid and no air before starting the engine, but I guess there is some air some where, I don't have ABS, I read in a previous post it could be a master cylinder defect but the only thing I don't understand the car had good pressure in the pedal before the calipers were replaced, so I have no idea on what to do, any help would be very grateful.



Is there a certain way I should have bled them? I did start to bleed the front two I started with the front right and my buddy said while it was running it was about two pumps and it built up good pressure, I then pursued to bleed the front left and then all the pressure was gone after I bled it, so as of now I am at a loss. Also I saw bubbles popping up in the master cylinder as the brakes were being pumped I don’t know if that is normal but its and observation.