1992 Factory Service Manual Brake Bleed Procedure

[Yellowoctupus]

BRAKE BLEEDING – MJ

A different rear brake bleeding procedure is required on MJ models.  This is necessary because of differential pressure valve internal circuitry and height sensing valve fluid line connection.

 

Essentially, the pressure differential valve inside the combo valve must be moved to a full forward position. This is necessary to allow complete bleeding of the combo valve, rear brake height sensing valve and both rear brake feed circuits. A manual bleeding procedure is preferred for MJ models.

 

Be sure to follow the MJ brake bleeding procedure carefully.  Incorrect brake bleeding will result in either a low pedal, or a low and spongy pedal

 

MJ BRAKE BLEEDING PROCEDURE

 

(1)    If master cylinder has been overhauled or a new cylinder will be installed, bleed cylinder on bench before installation. This shortens time needed to bleed system and ensures proper cylinder operation.

 

(2)    Fill master cylinder with Mopar or equivalent DOT 3 grade brake fluid.

 

(3)    Open all caliper and wheel cylinder bleed screws

 

(4)    Close bleed screws after fluid begins flowing from each screw.

 

(5)    Top off master cylinder reservoir again.

 

(6)    Apply parking brakes and shift transmission into Park, or Neutral, if equipped with manual transmission.

 

(7)    Open right front caliper bleed screw. This screw will remain open throughout rear brake bleeding procedure.

 

(8)    Position drain pan under right front caliper to catch brake fluid as it exits bleed screw.

 

(9)    Press brake pedal all way to floor and leave pedal in this position. This action will move pressure differential valve fully forward. 

 

(10)  Start engine and observe red brake warning light. Light should remain on after engine is running. This indicates that differential valve is fully forward.

 

(11)  Stop engine and fully release parking brake. Do not close right front bleed screw yet. Screw must remain open until rear brake bleeding is completed.

 

(12)  Observe flowing brake bleeding precautions:

·         Do not pump brake pedal at any time while bleeding. Air in system will be compressed into small bubbles that are distributed throughout hydraulic system. This will make a second and third bleeding operation necessary.

·         Bleed only one wheel brake unit at a time and use a bleed hose to bleed each wheel brake unit.

·         Attach one end of bleed hose to bleed screw and insert opposite end in glass container partially filled with brake fluid. Glass container makes it easier to see air bubbles as they exit the bleed hose.

·         Be sure end of bleed hose is immersed in fluid. Immersing hose end in fluid prevents air from being drawn back into cylinder and brake line.

 

(13)  Bleed left rear wheel first and right wheel second. Have helper operate brake pedal while bleeding each wheel.

CAUTION: Do not allow the master cylinder reservoir to run out of fluid. An empty cylinder will allow additional air to be drawn into the system. Check fluid level frequently and add fluid as needed during bleeding operations.

 

(14)  Bleed right front caliper first and left front driver side last.

 

(15)  Refill master cylinder if necessary.

 

(16)  Check brake operation before moving vehicle. Pedal should be firm, at normal height and not exhibit any sponginess.

[MiNi Beast]

Just what I needed to refresh my mind. Thanks for posting. 

[Eagle]

There are several potentially confusing typographical errors in the above. More important, it completely omits the part about resetting the shuttle and then bleeding the rear brakes a second time to bleed any residual air out of the normal side of the rear brake circuit.

[fiatslug87]

The 92 procedure with rebleeding  has been posted on here somewhere.

[Yellowoctupus]

Hi Eagle, Please let me know (PM or reply) where you saw any typos and I'll edit the post.  I transcribed it from a JPG posted here a few years ago that I could barely make out, so it's certainly possible I goofed something. 

 

Sorry fiatslug87 if it's a reposting, I kept finding various debated proven/misproven methods on bleeding I figured it would be good to have a 'sticky'd' post of the factory method to reference (whether it's the best/worst/omitting anything etc) as it seems to be a VERY common topic here. 

[gogmorgo]

11 hours ago, Yellowoctupus said:

Hi Eagle, Please let me know (PM or reply) where you saw any typos and I'll edit the post.  I transcribed it from a JPG posted here a few years ago that I could barely make out, so it's certainly possible I goofed something. 

 

Do you still have the JPG to add to the post? Even if it’s fuzzy, original sources are nice. 
 

The one glaring typo I noticed is in point 11, where you typed “ending” instead of I believe “engine”.

I think for the most part you got it down, it’s just that in typical service info fashion the method that’s different on the MJ gets obscured by all the generic bleeding instructions that most people familiar with brake bleeding will skip past. But it’s possible a couple lines got left out. I don’t really see anything about closing the front bleed screw and bleeding the rear a second time, which is the method that gets repeated on this forum. 

[fiatslug87]

The main confusion comes from the previous years FSMs not having the correct procedure.

 

 

 

[Yellowoctupus]

Quote

The one glaring typo I noticed is in point 11, where you typed “ending” instead of I believe “engine”.

 

So, if I "stop ending" I'd be starting?  Good catch, thanks.  Here's the 'original' photo from this post: 

 

Definitive Comanche Brake Bleeding

[fiatslug87]

Here are better pics.

[Ωhm]

Procedure states “Essentially, the pressure differential valve (inside the combo valve) must be moved to a full forward position”, but isn’t it shifted to full rear position, for complete bleeding of rear brake height sensing valve and both rear brake feed circuits. Maybe I’m just looking at the pictures wrong.

 

@Eagle

[Yellowoctupus]

The 'forward position' refers to the fact that the shuttle valve will actually move towards the front of the vehicle.  The right side of the picture is pointed towards the front of the truck.

 

[gogmorgo]

It's interesting. The process doesn't indicate that you should rebleed the rear circuit after closing the front bleeder.
Theoretically I suppose you shouldn't need to, considering the regular rear line isn't block off in the distribution valve and fluid should flow down both rear circuits at the same time and push air out of both. But I'd have to revisit the half-cut images of the load-sensing valve to determine if fluid pressure coming back past it could close the ports and block it... I wouldn't think so but I've been wrong before. It definitely doesn't hurt anything to bleed the rear a second time.
In practise having two paths from a single source to a single destination doesn't usually result in exact halving of the flow down each path, or even that one path will see any flow at all, so bleeding with the front bleeder open will get most of the air out and then closing and bleeding again will get it all out of the primary path. I can also see it being possible that if you have a fully bled primary circuit when you open up the bypass you'd end up with equal pressure on both ends of the bypass and simply not move anything through the bypass at all... hopefully the inner workings of the load-sensing valve would be enough restriction to reduce pressure at the outlet so stuff does move, and that restriction might be enough to get very little flow through the primary circuit when the bypass is open.

That was kinda rambly stream of consciousness but I think I just made a good enough argument for going one step extra on the FSM procedure and bleeding the rear brakes again after closing the front bleeder... It's five extra minutes and brake fluid is cheaper than sketchy brakes. 

54 minutes ago, Ωhm said:

but isn’t it shifted to full rear position,

 

Matter of perspective. It gets pushed towards the forward brake circuits. I think it is in fact shifted rearwards in the vehicle (I'd have to go open my hood to confirm orientation and it's dark out) but specific location in the vehicle isn't strictly critical knowledge to the bleeding process. 

[fiatslug87]

Here is the procedure from M.R.277 dated July 1985. Different bleeding order than the ‘92 procedure.

[gogmorgo]

7 minutes ago, fiatslug87 said:

Here is the procedure from M.R.277 dated July 1985. Different bleeding order than the ‘92 procedure.

Interesting. This is "bleed everything, open a front bleeder and bleed the rear, close the front bleeder and bleed everything again". I guess the initial bleed is for practise.

[Ωhm]

9 hours ago, gogmorgo said:

Matter of perspective. It gets pushed towards the forward brake circuits. I think it is in fact shifted rearwards in the vehicle (I'd have to go open my hood to confirm orientation and it's dark out) but specific location in the vehicle isn't strictly critical knowledge to the bleeding process.

 

I concur "location in the vehicle isn't strictly critical knowledge to the bleeding process.", but I wonder about whoever authored this procedure, is there anything else that could be in question.