Weak Breaks - Need Help Diagnosing

[dotnetrob]

My 1989 Jeep Comanche Pioneer 4x4. 4.0 has a brake problem.  

 

Back story

Got my comanche just over a month ago.  Brake were terrible, felt like rear brake were not engaging.  The bed load brake thing, I can't find the name for it, was broken and leaking, so don't believe rear brakes where getting much pressure/fluid.   Failed state inspection because of this.  Had a local show shop hard line the rear, so the bed weight brake thing isn't hooked up anymore, I also removed it from the bed.   Rear brakes where also worn out.   Had rear drums replaced and new rear pads installed.  They also replaced all the brake fluid, fluid appeared very dirty.   Got it back from the shop. Brakes felt a little better...  but still horrible.  

 

Brake pedal has alot of dead space / spongy before the brakes engage, feels like I am push 80% down before it engages.   Last 20% fells good, jeep stops, but after stopping it wants to lurch forward, keep having to push a bit harder to make it not roll forward.  

 

The previous owner did mention he replaced the master cylinder at some point.

 

Test Plan

Since rear brake are all new, I plan to start by looking at the front brake, maybe bleed the brakes myself to make sure there is no air.  Brake fluid reservoir is full and I don't see any leaks.  Usually spongy brakes have been air in lines issue or master cylinder issue, but I am no expert.  

 

Just looking for any advise/recommendations of things I can check to identify the issue and fix it.  

 

[89 MJ]

Did the shop change the distribution block for a proportioning valve when they deleted the load sensing valve?

[dotnetrob]

13 minutes ago, 89 MJ said:

Did the shop change the distribution block for a proportioning valve when they deleted the load sensing valve?

 

Looks like the brake line goes to a simple distribution block on the axle.  Took a quick picture.

[eaglescout526]

I had something similar to this. I ended up finding out after replacing my missing caliper pins that the pads were worn completely wrong and resulted in meh braking. I replaced the pads and solved my issue. 

[89 MJ]

5 minutes ago, dotnetrob said:

 

Looks like the brake line goes to a simple distribution block on the axle.  Took a quick picture.

This is correct and looks how it should. That hard line though on the frame side is interesting. 

The proportioning valve will be closer to the master cylinder and both front and rear brake lines will go into it.

[dotnetrob]

5 minutes ago, 89 MJ said:

This is correct and looks how it should. That hard line though on the frame side is interesting. 

The proportioning valve will be closer to the master cylinder and both front and rear brake lines will go into it.

 

I will trace the brake lines and see what I can find.

[dotnetrob]

12 minutes ago, eaglescout526 said:

I had something similar to this. I ended up finding out after replacing my missing caliper pins that the pads were worn completely wrong and resulted in meh braking. I replaced the pads and solved my issue. 

 

Inspection said front brake were at 12mm, but will be pulling it apart tonight to start checking myself.  Hoping it is just the front brakes.

[ghetdjc320]

3 hours ago, dotnetrob said:

The previous owner did mention he replaced the master cylinder at some point.

Can you take a picture of the booster and master cylinder? 

[dotnetrob]

2 hours ago, ghetdjc320 said:

Can you take a picture of the booster and master cylinder? 

 

Picture of booster and master cylinder attached.

 

5 hours ago, 89 MJ said:

This is correct and looks how it should. That hard line though on the frame side is interesting. 

The proportioning valve will be closer to the master cylinder and both front and rear brake lines will go into it.

 

Only other valve I could find is just under the master cylinder, traced all lines back to it and it is hooked to the master cylinder.  Tried to take a picture of it but its hard to photograph it.  There is a wire plug coming out of it.

[coolwind57]

Page 5 of one of my write-ups shows what I did to delete a few things.  Something for you to have a look at.  Might get you some ideas.  Good luck, friend.  It's about half way down page 5:  

 

[89 MJ]

Looks like you found what I was talking about. Does it match your brake lines in terms of aging and patina? Or could you ask the shop if they changed it?

[Pete M]

7 hours ago, dotnetrob said:

 

Looks like the brake line goes to a simple distribution block on the axle.  Took a quick picture.

 

 

that rubber line does not look long enough for full droop.   

[dotnetrob]

1 hour ago, coolwind57 said:

Page 5 of one of my write-ups shows what I did to delete a few things.  Something for you to have a look at.  Might get you some ideas.  Good luck, friend.  It's about half way down page 5:  

 

 

Great write-ups on what you have done to your comanche,  I like the changes you did to the brake line setup, I may attempt to do something similar.

 

10 minutes ago, Pete M said:

 

 

that rubber line does not look long enough for full droop.   

 

Yep, the rubber line will need to be replaced when I put on a lift, its way to short.

 

 

42 minutes ago, 89 MJ said:

Looks like you found what I was talking about. Does it match your brake lines in terms of aging and patina? Or could you ask the shop if they changed it?

 

Call the shop earlier today, they changed nothing up front, everything they did was in the rear, starting with the brake lines near the fuel tank and back.  

 

[Pete M]

I used a stock YJ rear line on my lifted truck.  plenty long enough.  

[89 MJ]

I’d suspect that the lack of proper proportioning valve is the root cause of your problems. I believe one from an XJ is the go to swap. 

[dotnetrob]

4 minutes ago, 89 MJ said:

I’d suspect that the lack of proper proportioning valve is the root cause of your problems. I believe one from an XJ is the go to swap. 

 

The valve just under the master cylinder looks alot like the XJ one I am seeing online.  It has some wires going to it.  Not sure if this is stock on the MJ or something that was added.  Not sure what the wires are going to, will try to trace them.   Could this be a proportion valve but maybe its not hooked up so I am not getting proper brake balance?  

 

[89 MJ]

It is likely just the factory distribution block. I do believe that they visually are very similar to the XJ ones, however the MJ one doesn’t actually determine the fluid that goes to the rear, that’s what the load sensing valve does. 
If the shop didn’t change it, you will probably need to. 

[89 MJ]

This might be worth the read. 

 

[AZJeff]

1 hour ago, Pete M said:

 

 

that rubber line does not look long enough for full droop.   

That rubber line is just barely long enough in the photo as is, where the vehicle is at normal static ride height.

 

the first time that axle is unloaded substantially, the hose will be damaged, possibly catastrophiccally.

[fiatslug87]

Here is the difference between MJ and XJ distribution valves.

photo credit to @Eagle

[ghetdjc320]

The major difference between the XJ and MJ units is that the MJ is just a distribution block and the XJ is a proportioning valve. Meaning the XJ unit will effectively reduce pressure to the back brakes. This is important in a disk/drum brake setup to prevent the rear drums from locking up prematurely. I’d try slamming on your brakes in a safe gravel area and see if the rear locks up before the front. Ultimately though, that little block helps you brake safely but shouldn’t greatly affect your braking power overall if plumbed correctly.

[dotnetrob]

Well started to pull the front brakes apart.  Front pads and rotors look great, almost like they are new and unused...  Everything else is rusty.  Decided to try bleeding the front brakes.  Looks like the brake bleeders on the front calipers haven't been touched in years and are rusted to the caliper.  So the shop was supposed to change out the break fluid definitely didn't bleed the front brakes.  Going to pull the calipers and repair/rebuild them and then re-bleed all 4 brakes and see if anything changes.

[ratrapp]

save yourself some work and just replace the calipers.theyre not super expensive and itll save you time and aggravation in the long run.if the bleeders snap off it a pita to get them out.

[AZJeff]

THIS IS SOLID ADVICE.  The cost of the parts, plus the possible discovery of pitted bores in the caliper body (and no way to fix them yourself) makes the rebuild of calipers at home an exercise in frustration.

[dotnetrob]

19 minutes ago, ratrapp said:

save yourself some work and just replace the calipers.theyre not super expensive and itll save you time and aggravation in the long run.if the bleeders snap off it a pita to get them out.

 

Yep, one bleeder snapped off, and the other side looked worse so I didnt even try.   Already ordered new calipers and a few other parts(pins, bushing, dust covers) that look worn out and old.