Brake issue PLEASE help

Hey guys ive got a 88 k1500 1 ton dually that has been swapped over to a 98, im having issues with the brakes on the truck. New master, and proportioning valve the 88 came with a abs modual for the rear ive since ditched this cuz it was junk so as of right now im running straight up non abs. Truck is running 98 motor and tranny along with the 98 ecu. Issue im having is the brake pedal is way to soft, and the rears lock up on me, everything is done no air in the system no nothing, i was told to adjust the rear shoes to help with the pedal not almost going to the floor when braking, but if i do that the rears are just going to lock up alot sooner i.e as soon as i touch the brake.
any help with this issue well be great! is i need this thing to pass safety a.s.a.p lol

If the rear brakes are way out of adjustment they will lock up instead of gradually stopping. If you have them in spec, and still lock up then you have a different issue. I would start there first. A perfectly working system with rear shoes way loose, can lock up. You may be looking further into it than need be.

ive been bleeding the brakes by pumping the pedal this whole time but i just went out and did a reverse bleed on it with a reverse bleeder pump, and the rear passanger side was easy going fluid pumped though nicely but the rear driverside was super hard to pump fluid though, any thoughts as why that is ?

The rear shoes being out of adjustment will most of the time give you a low pedal, not a soft one. Also getting low fluid flow from one rear wheel to the other could be a corroded and blocked brake line or a the same in the rear abs valve. You may be locking up the rear too soon because of the front brakes not working correctly or the rear abs system not working correctly allowing too much brake pressure to the rear. Are you running a factory hydroboost unit? If so make sure to bleed it correctly and that the power steering pump is working correctly "enough pressure" with clean fluid, any dark or burnt fluid could be signs of a bad power steering pump that can cause a world of problems including a low, soft and spongy pedal. I will PM you my number if you have more questions.

Alright so update! Well ive replaced the following
-wheel cylinders
-master cylinder (bench bleed)
-New proportioning valve
-all new lines with new spliters
With all that replaced im still at square one guys. The brake pedal still travels way farther then its suppost to and rear is still lockin up kinda like by braking is backwards
please help lol winter is coming very very fast and i need it ready asap lol any help will be great
thanks

could this be a brake booster issue?

Read this troubleshooting guide for low or no pedal, very in-depth guide for every scenario..........

ok i will read up on your link but i did go to the dealership and handed them the vin# for the truck and ive got one hell of a mutt on my hands. The truck started life out in 88 as a 4.3L v6 1/2 ton pick up truck !! So now since it has 1 ton axles in the rear do i run a whole half ton brake system like i am already or swap it out to the 1 ton brake system ?????

Just to check the obvious first....are you positive you have the lines connected in the right ports on the master ? The master not only has two sections but they are different bore diameters for disc and drum.....front port doesn't always mean front brakes, they are often backwards.

Trace the lines from the master to the destination, if they are reversed it will give the wrong pressure to the corresponding drum or disc.

No you don't need to swap to all 1/2 ton stuff, pressures in drum brake cylinders are very universal as are disc....they just need to be on separate circuits.

Did you pre adjust the shoes ? .....they need to be as close as possible to the drum with a very slight drag being ok. Also adjust up the e-brake after .......

Like krewzlo said the shoes to far out can grab aggressively when applied due to the leading edge digging into the drum.

Do you have the correct residual valves ? .....these are in place to keep some pressure in the line for the next braking cycle and keep a high and firm pedal. Drum is about 10psi...to compensate for the spring return pressure, Disc around 2psi....most are built in the master some are not.