Any chance your booster is bad? What kind of pedal effort are you having to use? The only time my brakes sucked on my 87 YJ when I had the stock axles was when my booster was leaking and I disconnected it so my motor would run properly (since it was basically a big vacuum leak). Even w/o the booster the brakes worked, but w/o the power assist I had to do a lot more work to make it stop. 33s shouldn't really be too much for reasonable stopping w/ stock brakes.
I know that you mentioned that you replaced the wheel cylinders, but is your brake adjuster on the rear wheels working? If it won't turn freely, it's not. Every time you back up in a vehicle with drums, it adjusts the pads to where they need to be. If the adjuster won't turn easily, your rear pads probably aren't hitting the drum.
You can buy a kit for the drums to replace all the springs and whatnot. If this has never been done, it's probably time.
I doubt it's your front brakes. When the calipers go bad, it's doubtful that they'll both screw up the same way at the same time and if just one is acting up it'll pull to one side like a mother...
The proportioning valve is a potential problem, but I'd be more suspect of the master cylinder (I've had brand new ones that didn't work on more than one occasion).
All of my brake components are lifetime warranty at this point. If something acts up, I pull all of it, take it in and get new ones. Master cylinder, wheel cylinders, calipers, shoes, and pads. On the Jeep, I've never had a problem with any of the other components in over 300k miles. I'm running 33/12.50-15 Hankook MT tires on Centerline Hellcat (aluminum) wheels and I have to be careful when stopping fast to avoid locking up.
Upgrading the brakes is always a good idea, but it sounds like you have a component problem and that needs to be addressed before you start throwing upgrades at it that may mask the real problem. The most likely suspects are either the adjusters or the master cylinder. While proportioning valves and boosters can fail, it doesn't happen often. Stick with the basics and go from there.
“The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed.” ― Robert M. Pirsig, Zen And The Art Of Motorcycle Maintenance: An Inquiry Into Values
Just an update.. I followed the advice and adjusted the back brakes as described. I then checked for vacuum leaks found one and repaired it. I also tested the brake booster as described and the pedal did fall but only about an inch or so. It did not fix the problem. Should there be a significant drop in the brake pedal when the vehicle is started?
The amount of pressure needed to press the pedal should drop a lot with the vehicle running vs. not. Even when I had a leak in my brake booster it still improved the braking effort, it just made the motor run terrible.
So do you have a spongy pedal or a hard pedal? Spongy with bad braking would indicate air in the lines. Hard pedal would mean a bad booster.
The pedal is pretty solid when the vehicle is turned off, when it's running it feels like there's no breaks at all.
Bleed the brakes again. Same thing happened to me, except I had my front callipers on upside down. So there was a bunch of air in the system. I went through 3 liters of fluid before I found out my mistake. Bleed them again. Make sure all the air is out. And also bleed them in order as well.
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