'91 Cherokee Brake Problems

[Bob]

I've had an ongoing problem with the brakes on my '91 Cherokee. It appears, over time, line pressure has diminished to the point that I can actually touch the floorboard with the brake pedal without locking-up the brakes. I have recently replaced every component with the exception of the porportion valve and front brake lines. The master cylinder & lines have been pressure bled at least three (3) times. The only non-Mopar components are the front pads and matching rotors--these are Raybestos PG-Plus semi-metallic's. After involving the dealer, they told me that all '91 Cherokee brakes are like this. I have a 3" lift w/31x10.5 BFG tires... but I don't think this would affect line pressure. I also don't recall my Cherokee doing this when I drove it off the showroom floor 10 years ago.

I would appreciate any expertise or advise before spending another $250 on a new porportion valve. My Mopar brakes are now worth over $2K!

Thanks in advance.
Bob

[XJPete87]

Have you replaced the master cylinder? The seals could be shot. YOu might also have a leak. Are you loosing fluid over a period of time?

[Bob]

Everything is new... Mopar parts... including the master cylinder, vacuum booster & check valve, rear wheel cylinders, drums, brake lining+hardware and front calipers.

The only new aftermarket parts are Raybestos rotors and pads... and I have been using these for 8 years with no problems.

There are no visable leaks or loss of fluid... the vacuum booster improved the situation but still has spongy pedal that wants to go to the floor when I really need the brakes (I am getting ready to buy an ARB Bull Bar to move anything that gets in front of me).

The Mopar/Jeep service manual doesn't elaborate with these symptoms... the only parts I haven't replaced are the proportion valve and front flex lines... I would only be speculating if these are the contributing factor to this problem---but these are another $400.

The only rational behind this would be the proportion valve is failing and sending random pressure levels to the front/back--I notice line pressure changes randomly. And a long shot is the front flex lines are getting old and swelling when I place extreme pressure on the master cylinder.

I don't like chasing hydraulic brake problems by just replacing parts... any insight would be appreciated.

[satan]

OK, the '91 brakes -- this is when DCX thought it'd be fun to reduce the rear drum size (did you end up with the 9.5" rear drums? - the previous years had 10.5x1.75" and they barely stopped, so reducing the diameter ... -- ahh! it hurts!)

Anyway, I digress...

Any swelling of tubing will cause a spongy feel (not the pedal flooring you describe), worth eliminating all posible causes as described. Air in the lines (I know pressure bled several times, corectly?) and/or poor quality fluid (all DOT 3s aren't the same) can contribute to softness, but not flooring unless you really SUCK at bleeding.

Your thought on the front lines is a solid one - if you can I would suggest trying to avoid the DCX hoses and go straight for teflon covered in braided stainless. The teflon lines will reduce the swelling even over new rubber hoses. If you are really hitting the floor with your pedal, the lines will not fix the issue (they would have to expand to take 50% of the designed front caliper travel volume - very close to 1/2 ounce -- that's a BUNCH). Still very worth doing - with 3" of lift (if you've changed the shocks and added real droop you'll need longer than Mopar XJ rear flex line as well - I know it's new, but if it's rubber it'll swell).

Are you positively sure that you have the right parts for your brake configuration - when DCX changed the rear brakes that changed the cylinders and master cylinder stoke volume?

When you replaced the rotors and drums, did you turn them and how were they cleaned prior to installation(turning will help to discover runout and out-of-round - either condition would increase the amount of fluid required to brake the vehicle, while the out-of-round would probably be felt as pulsating brakes, the runout issue is usually transparant to the driver)? Measure the front rotor run-out (also will indicate bearing issues -- something you probably didn't have 10-years back).

With the vehicle dead-assed cold, do any of the wheels present ANY brake drag (as in can you hear the shoes or pads giving a little scrape)? Is it periodic or constant? how much space is in between the pads and the rotor?

It would be worth measuring the rotor and caliper temperatures after along drive with minimal braking, then again after hard braking to see if there is a substantial increase in temp. (thought is that increased temp on the friction surfaces tends to degrade friction perfrmances and after time glaze/temper the braking surfaces further exacerbating the problem).

If your vehicle is still equipped with the factory proportioning valve, it can be dissassembled, inspected and reassembled (don't tell them that & be sure to re-bleed the entire system)-- BUT even with the plunger removed from the proportioning valve the brake pedal travel should not increase (effort and forces will change, but not stroke volume)-- Once the fluid volume is replaced, you will still be pressurizing the same system with the same properties (pump, pressure, swell, actuation) -- the proportioning valve would have to leak or swell to cause the pedal to floor.

Of course it's possible that you've received some of the wrong parts (thinking specifically the master cylinder and the rear cylinders - both are sized for the application, though you should've spotted the rear cylinders are "odd" straight away, they'd still bolt up)



<< CAUTION: SATAN'S RANT FOLLOWS >>

I will be the first one to say that the HO's experimental year (91) created some serious monsters, but the only real braking monster was the Bendix9 ABS system. My first guess (and forgive me, for I regularly see this) is that the system is (or was) out of adjustment at some time leading to where it is now --

Typically I see someone who has squealing or ineffective braking - they do a front brake job & things improve returning to poor braking in several thousand miles, then they look at the rest of the system doing rear brakes, lines, and master cylinders each time noticing some improvement.

Forgotten is the fact that fixing the front brakes never restored the factory feel of the system (did not fix the problem) , and after time seemed as ineffective as pissing into a fan.

In the XJ (renowned for it's perfectly ineffective rear adjustment system), what I see is that the rear brakes slowly loose efficacy as they wear and don't adjust. The extra braking load is moved to the front discs causing them to wear or heat to squeak -- People then fix the affect and not the cause (front brake job), then they promptly go out and glaze their front rotors and pads before noticing that the rears also suck (not thinking that that was the original causal effect for this sad degradation of braking performance).

Once the rear brake job is done, the factory feel is not restored because the front friction values are gone from the earlier abuse (even just a few hundred miles), and no one suspects that they've ruined their initial efforts so they look at the rest of the system.


As a professional mechanic (16-yrs was enough of that) , and a multiple XJ owner (finaly down to qty=3), that's what I see.

-- I have an '87 with 235s and 10.5" rear drums, which can lock all 4 wheels (if you can keep your head off of the glass), the other '87 has 33's and will slide sideways with excessive application of the brakes, and the '91 Loredo with the nasty ABS will put both of the '87s to shame in a short stop even with the crappy littel 9.5" drums in the rear.

[Bob]

Satan,

The answers to your questions:

1. Did you end up with the 9.5" rear drums? Yes.

2. I know pressure bled several times, correctly? I only watched them do this... the mechanic(s) appeared to know what they were doing. They used SOP to start from the farthest wheel and worked their way to the master C.

3. Are you positively sure that you have the right parts for your brake configuration - when DCX changed the rear brakes that changed the cylinders and master cylinder stoke volume?

Somewhat sure... you see... I originally took it to Big O Tire. They did the backs and fronts using the original calipers and cylinders (110K miles). My first mistake was similar to your comments in your extended "Rant". But, I soon knew a had a problem child and forced Big O to remove all Napa parts from my Jeep and replace with all new factory Mopar parts. The only thing I retained were the Raybestos rotors and pads---I didn't think this would be the issue. After they completed the job the pedal was the same---no improvement!

I finally took it to the dealer to verify I had the correct parts... but you make a good point about the stroke volumes. It almost appears this could be the problem... but my old master C did the same thing. Since 80% of stopping power comes from the front I my energy and $ wants to look there.

4. When you replaced the rotors and drums, did you turn them and how were they cleaned prior to installation?

I don't think they turned the new rotors or drums.


5. With the vehicle dead-assed cold, do any of the wheels present ANY brake drag? No. how much space is in between the pads and the rotor? None visually.

The only drag that started to appear after Big O's first attempt was I started to feel and hear a weird thump from the rear when coming to a complete stop, or after lifting the brake pedal from a stop---cold or hot. Originally I thought it was the old cylinders hanging up due to age/dirt. But now that I have all new parts I question if this is a related problem. It almost feels like it's coming from the diff or possibly the drive shaft yoke. What's strange is after I adjust the rear brake shoes the thump appears to diminish. The rear diff has made a clunking noise for a long time when downshifting. Other than that everything is solid.

<<<< TWO QUESTIONS FOR YOU >>>>>

If the proportion valve was faulty, could it possibly redirect more pressure to the inferior 9.5" rear brakes where only 20% of the stopping power comes from?

If yes, could this potentially affect overall stopping power... ultimately allowing the pedal to hit the floorboard due to lack of pressure to the front calipers where 80% of the stopping power comes from?

I really appreciate your input... this leaves me with many more possibilities to explore.

Bob

[satan]

Sorry about the 9.5 drums... Very!

Yep it's always best to look at the front...

... in that light - you mention

The only thing I retained were the Raybestos rotors and pads


Were these rotors ever cleaned-up (turned) and/or the pads exchanged? (thinking that your issue is following these parts -- they were installed to solve an issue, then beacame ineffective..? and are still on there?)

The thump you describe is consistant with the rear shoes having to move too far (and releasing too far). Adjust the rear shoes AND the ebrake cables (since those systems are used to set the reference points for shoe travel). Still wouldn't cause your pedal to Floor, but could contribute to "hard" or "soft" pedal - depending on how far things move.

<< the questions>>

A bad proportioning valve would not let the pedal move more, they tend to fail as you describe allowing pressure to divert in oone direction over the other--- Considering that the rear cylinders are a fraction of the volume of the front caliper bores, if your valve failed in such a manner your pedal would barely move and your rear brakes would be VERY VERY hot after normal driving (check em!)...

The pedal would be firm and the braking would be horrible.

Something that you can do to Check/play with the proportioning valve is detailed here (http://www.bigoffroad.com/pages/bigpages/bigtech/modified_proportioning_valve.htm). The article is kinda OK and is intended as a stop-gap measure for where rear discs have been installed requireing add'l fluid volumes. The net effect of the described modification is to defeat the "proportioning" part of the valve and apply essentially equal pressures and flow rates to the front and rear lines. You will need to re-bleed your brakes after messing with the valve.


the thunk has me intrigued -- many of the late model 9.5" drum axle owners have commented on a driveline thunk... It would be interesting to pull a wheel and drum off and measure the ID of the drum and contrast it with the OD measurements of the shoes (I'll wager that the shoe shape is far more ovoid than expected leaving odd-sized gaps between the friction linings and the drum's inner surface!)

[Bob]

Satan,

This is long so bear with me...

Based on your summary, I don't think my proportion valve is faulty. The article you mentioned sounds like the same problem with my '65 Race Hemi clone when it was built. The front drums were converted to discs and needed balancing due to misallocated line pressure.

You asked the question:

[QUOTE]
Were these rotors ever cleaned-up (turned) and/or the pads exchanged? [END QUOTE]

No, I thought about insisting they (Big O) pull and turn the new RayB rotors and put in new RayB pads but they were already working for free. My thoughts were to try and isolate the real problem---maybe I did by screwing around with the master C! And, the only reason I changed that out was because it showed signs of seepage around the vacuum booster---but that could have been cause by spilling brake fluid when they bled the system the first time.

I don't usually like buying aftermarket brake parts, but in '91-'94 the OEM pads were only good for 15-20K miles... and then I would find warped rotors to-boot. Finally, after two sets of rotors, I specified RayB semi-metallics with OEM rotors---this worked perfect for years. Then recently, when the rears finally went bad I spec'd RayB non-metallic for the rear---thinking this would work with the OEM drums. And they may have with the right combination. After all, I used RayB's for 8 years.

However, recently, I heard that some aftermarket rotors that are coming from offshore sites are NOT always made to the same specs as OEM's... and could cause poor braking. I wouldn't think RayB's are in that category... that's why I didn't balk. The only other possibility is the front pad/rotor combination may be too hard of a composite for this application---I sent an email to RayB but received no response.

>>>>>>A POSSIBLE POOR COMBINATION<<<<<

[QUOTE]
the thunk has me intrigued -- many of the late model 9.5" drum axle owners have commented on a driveline thunk... It would be interesting to pull a wheel and drum off and measure the ID of the drum and contrast it with the OD measurements of the shoes (I'll wager that the shoe shape is far more ovoid than expected leaving odd-sized gaps between the friction linings and the drum's inner surface!) [END QUOTE]

I believe this is happening... I removed the right rear about a month after all these parts were swapped-out. Aside form the fact that the rears were not adjusting properly... I noticed the top of the shoe was ruff and chamfered... thus supporting your theory.

I haven't done a brake job for over twenty years and you know, I felt like a dumb s..t when I asked them if they arc'd the rear shoes... They told me that's old-time stuff and nobody does that anymore. Later, I found out why... environmental issues!

>>>>MY CORRECTIVE ACTION<<<<

I think I am going to do the following:

[QUOTE]
1. Adjust the rear shoes AND the ebrake cables (since those systems are used to set the reference points for shoe travel). Still wouldn't cause your pedal to Floor, but could contribute to "hard" or "soft" pedal - depending on how far things move. [END QUOTE]

2. Change out the front rotors and pads to the new Mopar "V Line" OEM replacements.

If this doesn't work, then I am adding more lift and will swap-out the rubber lines with longer braided versions. Oh... I forgot... and a new ARB Bull Bar in case anyone gets in front of me!

Thanks for the insight!
Bob

[satan]

Sounds like a good path and then you'l know much more about what's doing what where - Sounds like some of the pedal travel is the slack coming out of the rear, and then maybe a ,ittle glazing on the front and you'd have some crappy brakes...

... you may want to add a skid-plat or two when you get the bullbar. It'll be easier on the oilpand and transmission if your run up-n-over whatever stops in front of you!

Let us all know how it goes.!