Weak brakes 61 c10 disc upgrade

[buds56]

I have a 61 chevy tk project that I converted to front discs with an 87 front crossmember, also installed the later drum brake rear out of the same 87 parts truck.
I used a new mid 70's impala booster and 1-1/8" bore master cylinder, both reservoirs are the same size. Capt.Fab bracket.
New calipers, hoses, wheel cylinders, shoes, steel lines, drums are within spec rear shoes adjusted up.
The proportion valve is out of the 87 parts truck. pedal is stock 61.
I got it running and road tested it and the brakes feel weak, the fronts will barely lock up on gravel and the rears do not on gravel but do lock up in the air
I have bled the brakes a couple times and am getting good fluid out all the bleeders.
I adjusted the rod from the booster into the rear of the master cyl to take up the slack, which did bring the pedal up, but no improvement in stopping performance.
This 87 had one of those plastic aluminum master cylinders and from what I've read they have a 26mm/40mm bore (1-1/8 - 1.57") I'll admit I don't know much about these.
Are the proportion valves different on these later square trucks?
Should I get rid of the factory valve and put in an adjustable one?
In my mind this all standard 71-87 GM stuff and should work
Any help or ideas would be appreciated.
Thanks,Bud