Hey guys,

So before I get slammed for putting this under a new post- I've already searched and I'm just generally bewildered..

I'm having serious brake problems on a generally well kept 1988 xj, 4wd, 4.0. It's mostly stock, but I've replaced a lot of parts due to age.

I'm gonna abbreviate this post, because if I listed in order the whole ordeal with my brakes, it would be too long. Basicly I've replaced almost everything except the proprtioning valve, the soft rear rubber line to the splitter (nuts are rounded off), and the hard lines. Every other component has been replaced- literally, calipers, rotors, front soft lines, new bleeder screws, while rear drum assembly, including shoes, pads, wheel cylinders, and springs. Additionally, I've replaced the booster and master cylinder no less than 4 times, with different t parts from different companies, Autozone, ac Delco, and crown automotive.

Ive destroyed a vacuum bleeder bleeding my system, ive literally put gallons of dot 3 through the system, mc has been bench
Bled, there are no leaks I can find at any of the fittings, and fortunately/amazingly my xj has no rust on it.

After having replaced everything so many times- I finally took it to a mechanic- because I still didn't have any brake assist, unbelievably- I thought maybe it doesn't have enough vacuum, I'm getting about 18 at sea level. No vacuum leaks. So I said hell with it- let someone else look at it.

So it's at the shop forever and I'm having jeep withdrawals, he finally calls me up, "well it's stops but there's something wrong I can't figure out- the mechanic is telling me this- so the mechanic explains- I've replaced the booster, and the master cylinder with a brand new not man master, bled the lines repeatedly- definitely no air in the system, the rear drums are adjusted properly, push rod for the booster:mc adjusted properly, no apparent leaks around any of the fittings or at the wheels, and the fluid comes out clear.

The symptoms I'm still having- soft pedal- goes all the way to the floor. No hissing when brakes a applied, no fluid leaking in the cab at the pedal, the pedal won't "pump" up its soft all the time, and it stops consistantly at the last 10-20% of brake pedal travel. It will almost lock the wheels up if you push all the way, the nose dives appropriately.. I'm at a loss as to what's wrong. The mechanic says there's good even hydraulic pressure at all 4 wheels. This system is an 88 renix- no abs.

Any ideas? Please help

 

You need to replace the rear soft line.

 

At this point I'd get some threaded plugs and cap off the proportioning valve port that feeds the rear brake lines. If that stiffens up the pedal you know the problem is behind the prop. valve.
If the problem persists, cap off the port that feeds the front lines.
If the problem persists with both ports capped, it's the MC.
At least you'll know which portion of the system is at fault.

 

first of all, the rear soft brake line should be replaced, its about time

second, even after rear soft line replacement it doesn't work as it should, get a set of brake hydraulic pressure test gauges, you need two to monitor front and rear at once.

check the manual for specs, if the rear is off - proportioning valve has to be changed OR

if you think that this process may end up costing too much, change the proportioning valve with a brand new part without any diagnosis.

However, a soft pedal is mostly caused by a bad rubber seal in the MC, old brake soft lines, air in the system or a leak.

Also, I have seen that bleeding vehicles with their rear slightly jacked up so that the MC is straight (best checked by a level meter) helps a lot.

EDIT: Radi proposed a simpler way to rule out or in proportioning valve / MC

 

Check the length of the booster pushrod as it protrudes into the master cylinder. It's adjustable.

 

Thanks for the replys, it's been storming like crazy all week, I'll get out there and start tinkering soon and get back to you with my findings!

 

Note from DJ to OP Nicholas
I must ask you this question when you replaced your front calipers you did windup with the bleeders on the top I hope; you would not be the first to reverse the sides on the calipers and end up with the bleeders on the bottom?

 

X2 on this. I had one that would bulge only when you mashed down on the pedal. Made the brake pedal feel soft as it was "inflating" the tube.

 

There is a proportioning valve that is attached to the rear axle and the truck body with a linkage. It diverts more brake fluid pressure to the rear wheels when the truck bed is loaded. It's located just to the rear of the rear axle, near the center of the truck. Check to make sure that this linkage is still attached. I don't know is this will cause your particular problem, but it's worth checking.

 

Not on xjs just mjs.