my abs went out so I deleted it with a proportioning valve.
Not the best possible solution. Even an entire combination valve would be a less than optimum.
Obviously not.
at a red light it's as if I'm pushing the gas pedal, I have to put all my weight on the brake pedal and the truck still inches forward...
...New brake booster, new gmt800 cylinder, new calipers, everything in rear drums new, new rubber hoses on the front, rear still has old hoses but basically my entire brake system is new besides that
1. WHICH "new brake booster"?
2. The GMT800 master cylinder is almost certainly a mistake of great importance.
3. WHICH "new calipers"? Low-drag?
4. Are the rear brakes properly adjusted? Does the park brake work, and do you use it frequently?
5. How many "old hoses" in the rear? Do the rear brakes bleed nicely, good volume of fluid out the bleeder valve?
6. What is the idle speed of the engine?
7. Are you running oversized tires?
Replacing everything at once makes it almost impossible to find the problem.
Certainly makes it more difficult, anyway.
Why the GMT800 cylinder? Was it "bench bled"?
Bench-bleeding a master cylinder takes more time and effort than most folks realize.
I got rid of the proportioning valve in favor of a 10# residual pressure valve.
Also not an optimal solution, particularly when braking hard. Did you disable the brake-pressure warning light on the dash? Did you remove the holdoff valve that balances front/rear brake pressure when the brakes are first applied?
Its been proven here that if you just do the GMT800 master with GMT400 other components it'll be worse than the 400 master. You can either upgrade calipers and wheel cylinders to GMT800 or put a GMT400 master back on it.
True enough for any low-drag GMT400 calipers. Less true for the JB/JD7, 8, and JF9 calipers.
But yes, the larger bore of the GMT800 master cylinder isn't helping any, the hydraulic advantage is lessened so there's less brake pressure holding the brakes on.
The 400 master will be the cheapest and easiest option for sure. And yea, be sure to bench bleed it thoroughly before installing. It makes a difference.
Absolutely. We're probably looking at the 1 1/8" bore master, with the 40mm Quick Take-Up bore.
There's a smaller master cylinder for the older JN3 low-drag calipers.