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Soft brake peddle cause

I just went through this kind of nightmare with our 92 xj. Booster went out and I decided to replace the booster & MC, then one of the lines mc to proportioning valve broke (btw advance auto has a straight line with the proper fitting all it needed was to be bent to shape).
We had a ton of air it took a some time to finally get it all out. Petal still felt off so it ended up needing adjusting at the back drum. Adjuster don't seem to work.

But on this one my first guess would have been the MC. It sounds like you replaced about everything that could be the problem.
Can the proportioning valves go bad or leak?
 
I just replaced mine and a leaking line from MC to combo valve and now I have reasonable brakes with ZJ MC and booster. I don't think the line was leaking until I started putting everything back together and was twisting the flexible line. I think I broke it loose at the connection so I replaced both flexi lines with solids while I was there. Haven't taken the old combo valve apart yet but I suspect it was as gunky as my MC that I had to replace because I just couldn't get it clean enough.
 
Personally, I have not used a vacuum to bleed brakes nor have I ever seen anyone use it, but,...... is it normal to vacuum the brakes through the master cylinder. My thoughts is to use it in the other direction i.e. attach the hose to the wheel bleeder and pull all the air down and out of the system.

In professional maintenance shops a low pressure of not more than 10 PSI is inserted into the master cylinder forcing the air laden brake fluid out the bleeders. The wheels are bled in the normal way doing the wheel furthest from the master cylinder first gradually working up to the wheel closest to the master cylinder,....but of course everyone on the forum know the proper bleeding procedure.
 
Actually I was thinking of a similar solution only pressurising not vacuuming.

I have a tool (for other vehicles) that is basically a pesticide or garden spray bottle, it has a fitting the perfectly fits the brake mater cylinder reservoir. Add brake fluid pump it to pressurise and Voila.

There is another tool that looks like you pressurise a bottle with a air compressor use a fitting over the reservoir and pressurise it that way. I've only seen picture so I'm not sure about the second tool.
 
Today I helped a buddy replace all the brake lines (hard & soft lines + new calipers/pads/rotors) on his 00 XJ after we tightened everything up we used an empty Motive pressure bleeder to check for leaks, pumped it up to 15psi & used leak-tec (basically soapy water) to look for leaks, I was amazed at how tight 2 of the banjo bolts had to be to stop the leaking air completely.
Have you air leak tested the system?
 
garr said:
Why do most say to never go higher than 15-20psi pressure when air pressure testing a brake system or when using a power bleeder?
Click to expand...
That's fine to bleed the system of air but a poor procedure to check for leaks. That must be done under full pressure. i.e. someone standing on the brake pedal as hard as they can with the engine running.
 
Digger87xj said:
That's fine to bleed the system of air but a poor procedure to check for leaks. That must be done under full pressure. i.e. someone standing on the brake pedal as hard as they can with the engine running.
Click to expand...

Then why not just hook up the shop air, dial up the air compressor regulator to 200psi & check for leaks?
 
garr said:
Then why not just hook up the shop air, dial up the air compressor regulator to 200psi & check for leaks?
Click to expand...

I think you would mess up the reservoir. The plastic isn't under pressure like the rest of the hydraulic system. Now you can pressure test it in a more standard way if you can get a gauge on the MC side of hard line and pressurize the line, wait a bit and see if the pressure drops.
 
If you put 200 PSI air pressure on the reservoir, one or more of the following would happen
1, the reservoir breaks or pops off and brake fluid goes everywhere
2, the compressed air manages to aerate the fluid badly. I don't know why this happens but I seem to recall that it does.
3, somebody cracks a bleeder & blinds themselves with a high pressure jet of fluid.
 
yossarian19 said:
If you put 200 PSI air pressure on the reservoir, one or more of the following would happen
1, the reservoir breaks or pops off and brake fluid goes everywhere
2, the compressed air manages to aerate the fluid badly. I don't know why this happens but I seem to recall that it does.
3, somebody cracks a bleeder & blinds themselves with a high pressure jet of fluid.
Click to expand...

I'm hoping if you are pressure testing that you are doing it without fluid and doing it with air only. Not a good test otherwise. Still pointless in this scenario since honestly, with a little patience and proper installation of proper parts and bleeding, this issue goes away.
 
Not to mention what a waste of time and money trying to adapt air lines to the hydraulic system.... when all you need to do is step on the pedal and look for seepage.
 
dan1977p said:
I'm hoping if you are pressure testing that you are doing it without fluid and doing it with air only. Not a good test otherwise. Still pointless in this scenario since honestly, with a little patience and proper installation of proper parts and bleeding, this issue goes away.
Click to expand...

Yes the system is completely dry, new everything & I'm not trying to adapt anything, I simply pumped up my motive pressure bleeder to 15psi & let it sit for a few hours before the initial filling & bleeding of the system, the system is now full & bled, the motive pressure bleeder is a great tool.
 
I always had good luck with a hand vacuum pump at the wheel cylinders. You must start at the furthest from the MC, and the secret is not to worry at all about whether there's air in the line from the bleeders. Just keep sucking until the flow is even, and then suck out a little more. If you're not too conservative about saving an ounce or two of fluid, it works fine.

I have had problems with the flexible hoses looking good and expanding internally, which I presume Ecomike has already addressed, but for those fighting odd brake behavior, it's worth replacing any old hoses. They can also delaminate inside and behave like a one-way valve, and cause calipers to hang up.

Finally, remember that play in a wheel bearing can cause mushy brakes too, as even a slight wobbling of the rotor will push the caliper back further.
 
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