Brakes 78 CJ7 - rear brakes approx 10% power

[sewe]

Hi,
my rear brakes do not really have huge function (11" drum brake). If I push my brake pedal almost complete down to the floor the rear brakes begin to work a little bit. On the brake test station this gets up to 10% of the normal function...but then the floor does not allow me to push further.
In the same time the front brakes seem to work o.k. (not the best brakes but really o.k.) (front is disc brake)
I changed the master cylinder (new replacement part)
I changed all hoses and tubes
I changed the proporting valve
so what is still old: the rear drum brakes and cylinders in there.

if I push the pedal more often in really short time it works good
if I press the parking brake a little bit until it is beginning to brake and then use the normal brake pedal all is good (perfekt)

so I tried all I could think of in the last month...but I can not find the real problem(reason)

- so the first question I have: which would be the correct brake cylinders for the 11" drum brake (name, part number, diameters)...I am not sure if there are the correct parts mounted inside
- is this problem known in some way? any ideas?

what kind of information do I need to post here or update so that someone could help better?

CJ7
'78
front brakes: disc (6 holes)
rear brakes 11" drum brake
brake booster looks bigger than the original one...seems not to be the original one
brake main cylinder: replacement-spare part...but from here on all looks original
left and right brake are almost similar (bad) funktion (no real differences)

thank you very much!!
sewe

 

[Patrick]

I don’t think you have a wheel cylinder sizing problem. But you say you can put your brake pedal down to the metal. That is not normal.

 

[Armyvet25]

Bleed your rear brakes and see if you have air in the lines, at the same time note if you have a good stream of fluid, also, check rear shoe to drum adjustment, if too loose the fronts will damn near stop the vehicle and the pedal will travel to far before the rear brakes aren't effective exactly as you are describing.
lastly, possible defective master, if the internal cup seals aren't working correctly, the rears may not be getting the necessary pressure.

 

[TSB8C]

Since you say pressing the parking brake a bit before then pressing the brake pedal works "perfekt", it sounds like one of two things. Either your brakes need adjusting to be closer to their contact point, or the wheel cylinder is not pressing them out far enough. An issue with the wheel cylinder seems unlikely, so focus on adjustment. Using the adjuster wheel, spread the shoes apart until they just start to make the slightest contact with the drum when you turn it by hand.




Now you could have other issues since you ay that pressing the brake pedal a number of times close together seems to make them work better. That is usually a sign of needing to bleed air from the lines. Another possible issue, but less likely, is with the proportioning valve (off-centered plunger perhaps). But an issue there would normally trigger the brake warning light in the dash. And of course you already said you changed that, along with a new master cylinder.

You mention the booster seems bigger and/or not original. But a booster problem would affect all brakes, not just the rear. And if the jeep originally had manual brakes, someone may have put an after-market kit to convert to power. I did that and the kit I used had a dual-diaphram booster (PB-8531), so definitely larger than an original stock one would have been.

The correct wheel cylinders for 11" brake shoes are API 2202-07359910 (left side) and 2202-07702068 (right side). If someone used a set for 10" brakes, that could be an issue. I don't know if the mounting bolt holes are spaced the same on the ones for 10" vs. 11", so no idea if someone could have installed the wrong ones or not.

Good luck!

 

[CJ]

Just to add to the above information, I would agree to adjust your drums.
The brakes are actually self adjusting but most drivers do not know they only self adjust while braking in reverse.
To adjust them without crawling under the CJ, hit the brakes HARD while backing up at a speed of about 3-5 mph. NOTE: This will only work if your brakes are assembled correctly and your star wheel is not bound up. Do this several times and don't worry as they cannot be over tightened.

Or, crawl under and grab your adjusting tool...

 

[sewe]

Bleed your rear brakes and see if you have air in the lines, at the same time note if you have a good stream of fluid, also, check rear shoe to drum adjustment, if too loose the fronts will damn near stop the vehicle and the pedal will travel to far before the rear brakes aren't effective exactly as you are describing.
lastly, possible defective master, if the internal cup seals aren't working correctly, the rears may not be getting the necessary pressure.

Hi Armyvet25,
thank you for your help.
but the master brake cylinder is new...the problem was the same with the old one...
I bleeded the brakes with 2l of brake fluid...no air...all perfect

 

[sewe]

I don’t think you have a wheel cylinder sizing problem. But you say you can put your brake pedal down to the metal. That is not normal.

Hi Patrick,
thank you for your help. I also think that the cylinder sizing is not the problem...but I want to check all things...just to be sure...because I invested lots of time alreasy without any positive result.

 

[sewe]

Hi TSB8C,
thank you for your help!
I tr to add some comments below:

Since you say pressing the parking brake a bit before then pressing the brake pedal works "perfekt", it sounds like one of two things. Either your brakes need adjusting to be closer to their contact point,

I also thought so...but I adjusted the brakes so close to the drum that I could not turn the drum any more by hand...did not work as expected...was still bad. I drove some meters and the brake really got hot...I think I should not adjust it so close...

or the wheel cylinder is not pressing them out far enough.

this is the reason for asking if it is the correct one...but the cylinder is pressing far enough...but much too late...the cylinder is starting to move when the brake pedal is pressed 75% already...before this...nothing moves here (I opened the drum brake to see this)

Now you could have other issues since you ay that pressing the brake pedal a number of times close together seems to make them work better. That is usually a sign of needing to bleed air from the lines.

I also thought so...but I could not find any more air...I gave up with this issue...

Another possible issue, but less likely, is with the proportioning valve (off-centered plunger perhaps). But an issue there would normally trigger the brake warning light in the dash. And of course you already said you changed that, along with a new master cylinder.

I have no working light for the off center plunger ... the connector does not fit...so I could not attach it.
But I disassembled the proportioning valve again...and the plunger is 100% in the middle (still after disassembling...so I think it did not move...it is not moving easyly...so I think it was in the middle all the time)

You mention the booster seems bigger and/or not original. But a booster problem would affect all brakes, not just the rear. And if the jeep originally had manual brakes, someone may have put an after-market kit to convert to power. I did that and the kit I used had a dual-diaphram booster (PB-8531), so definitely larger than an original stock one would have been.

totally agree

The correct wheel cylinders for 11" brake shoes are API 2202-07359910 (left side) and 2202-07702068 (right side). If someone used a set for 10" brakes, that could be an issue. I don't know if the mounting bolt holes are spaced the same on the ones for 10" vs. 11", so no idea if someone could have installed the wrong ones or not.

thank you so much for the correct numnbers!! I will check them on the weekend...

best regards
sewe

 

[sewe]

Just to add to the above information, I would agree to adjust your drums.
The brakes are actually self adjusting but most drivers do not know they only self adjust while braking in reverse.
To adjust them without crawling under the CJ, hit the brakes HARD while backing up at a speed of about 3-5 mph. NOTE: This will only work if your brakes are assembled correctly and your star wheel is not bound up. Do this several times and don't worry as they cannot be over tightened.

Hi CJ,
thank you for your help!
I think your post could bring some new tests to me...but I do not get all of your words correct...is really hard to translate...so let me summarize to doublecheck:
I really did not know that the CJ7 has self adjusting brakes while driving backwards! But I even did not know the brakes are self adjusting anyway - really thought that this would be no feature here...so how could I really confirm that the brake is self adjusting? is there some part inside which needs to be included so I can really confirm this?

what do you mean with "star wheel"?
and what do you mean with "wheel is not bound up"

maybe this is one big problem that I do not understand the working principle of the rear drum brake totally...For example I have no idea what the metal wire is really doing...And I really have no idea how the self adjusting should work...I can adjust at the bottom of the brake with the adjusting wheel...but at the upper side where the cylinder is doing his job the distance to the drum is not influenced here...the distance at the top position (cylinder) will stay the same (long) way as before!?

best regards
sewe

 

[TSB8C]

The "star wheel" is the adjuster I showed circled in my earlier pic with the red circle near the bottom. Here it is again. And "not bound up" refers to the adjuster being free to move as it adjusts itself while stopping in reverse as CJ noted. Many times that adjuster gets rusted, or gets crud in it (build up of brake dust, etc) and it can no longer move freely. If you disassemble it and clean it out and add a small touch of grease inside the opening where it spins, the self-adjusting part will work much better.



Now since you said that you removed the drum and watched the wheel cylinder for movement while pressing the brake pedal, and you saw no movement until you had the pedal about 75% down, that certainly sounds like the master cylinder. Even if you had the wrong wheel cylinders in there (10" vs. 11"), the plungers on the wheel cylinder should be pushing outward when you press the brake pedal, and that movement should occur before having to push the pedal so far down. So perhaps the master cylinder does have an internal issue with the seals that the section for the rear brakes is not forcing fluid down as it should. To test this, you would need to open a brake line at the rear (either at the center junction block on the rear axle or at one of the wheel cylinders) and then check for fluid pressure while the pedal is being pressed. Then of course you'll need to bleed the lines again after you put it all back.

And BTW - Sounds like you replaced the proportioning valve with a PV2 (one pin plug) where as the original was a PV1 (two pin plug). They work fine, but the connector for the brake warning light is different. Here's a pigtail connector you can splice on to have your warning light again: Connector

 

[dmcguire]

I wonder if the brake pushrod is correct? Seeing as you had the problem before, and said it had a replacement Master, it may be the pushrod is too short. Too long would lock up your fronts after a short while of driving as it would not return and allow the fluid to depressurize. So I would think maybe too short of a pushrod would get you the mediocre front brakes and finally some rear.

Do you have a lot of play/rattling in the pushrod that goes into the MC? If it's adjustable, try tightening.

 

[TSB8C]

I wonder if the brake pushrod is correct? Seeing as you had the problem before, and said it had a replacement Master, it may be the pushrod is too short. Too long would lock up your fronts after a short while of driving as it would not return and allow the fluid to depressurize. So I would think maybe too short of a pushrod would get you the mediocre front brakes and finally some rear.

Do you have a lot of play/rattling in the pushrod that goes into the MC? If it's adjustable, try tightening.

That is a great thought. When I converted from manual to power brakes, my kit came with an adjustable push rod from the pedal into the rear of the master cylinder. Took a bit of trial and error to get the fit right. I also made a tool to measure and ensure that the "spacer" between the master cylinder and the booster was also the correct the length as it is adjustable too. Such tools can be purchased online, but making one is easy and cheap.

 

[Armyvet25]

I wonder if the brake pushrod is correct? Seeing as you had the problem before, and said it had a replacement Master, it may be the pushrod is too short. Too long would lock up your fronts after a short while of driving as it would not return and allow the fluid to depressurize. So I would think maybe too short of a pushrod would get you the mediocre front brakes and finally some rear.

Do you have a lot of play/rattling in the pushrod that goes into the MC? If it's adjustable, try tightening.

ding ding ding, sounds like we might have a winner there Monty, tell him what he wins...

 

[j6824]

Just a thought...might be worth rolling around:

https://jeep-cj.com/community/threads/amc-20-disc-brakes.61955/

 

[sewe]

Hi TSB8C,
thank you!!

The "star wheel" is the adjuster I showed circled in my earlier pic with the red circle near the bottom. Here it is again. And "not bound up" refers to the adjuster being free to move as it adjusts itself while stopping in reverse as CJ noted. Many times that adjuster gets rusted, or gets crud in it (build up of brake dust, etc) and it can no longer move freely. If you disassemble it and clean it out and add a small touch of grease inside the opening where it spins, the self-adjusting part will work much better.

now I got it...thanks! So this wheel was good all times...and I already did disasemble it and add grease...but it was also perfect before

Now since you said that you removed the drum and watched the wheel cylinder for movement while pressing the brake pedal, and you saw no movement until you had the pedal about 75% down, that certainly sounds like the master cylinder. Even if you had the wrong wheel cylinders in there (10" vs. 11"), the plungers on the wheel cylinder should be pushing outward when you press the brake pedal, and that movement should occur before having to push the pedal so far down. So perhaps the master cylinder does have an internal issue with the seals that the section for the rear brakes is not forcing fluid down as it should. To test this, you would need to open a brake line at the rear (either at the center junction block on the rear axle or at one of the wheel cylinders) and then check for fluid pressure while the pedal is being pressed. Then of course you'll need to bleed the lines again after you put it all back.

so I will order some measuring devices...I also have no other idea...will need 2-4 weeks...
but I have the same issue with the old and new main cylinder...so in between there is only the proporting valve...So I will check the preassure as soon as I got the measuring equipment.

And BTW - Sounds like you replaced the proportioning valve with a PV2 (one pin plug) where as the original was a PV1 (two pin plug). They work fine, but the connector for the brake warning light is different. Here's a pigtail connector you can splice on to have your warning light again: Connector

thank you! this connector is exactly what I need...but I will first fix the rest then order this one...

 

[sewe]

I wonder if the brake pushrod is correct? Seeing as you had the problem before, and said it had a replacement Master, it may be the pushrod is too short. Too long would lock up your fronts after a short while of driving as it would not return and allow the fluid to depressurize. So I would think maybe too short of a pushrod would get you the mediocre front brakes and finally some rear.

Do you have a lot of play/rattling in the pushrod that goes into the MC? If it's adjustable, try tightening.

Hi dmcguire,
thank you.
I never checked this idea...so I will also check it...but without knowing the correct dimensions this will get hard to know. At the moment all looks good here for me...but I will check all ideas! give me some weeks now...need to get a free place on the garage again...

 

[sewe]

That is a great thought. When I converted from manual to power brakes, my kit came with an adjustable push rod from the pedal into the rear of the master cylinder. Took a bit of trial and error to get the fit right. I also made a tool to measure and ensure that the "spacer" between the master cylinder and the booster was also the correct the length as it is adjustable too. Such tools can be purchased online, but making one is easy and cheap.

I will try!! sure!
but one thinking from my side: why is it getting good when the rear brake is pre-fixed by the hand brake...but sure I will check this point!!

 

[sewe]

Just a thought...might be worth rolling around:

https://jeep-cj.com/community/threads/amc-20-disc-brakes.61955/

I checked the thread here...so I have the correct proporting valve for disc/drum...but sure...this valve is still some big black box for me...

 

[sewe]

Thank you all for the great feedback!!!!!!
now it is up to me to test all these things...please give me some time!

 

[sewe]

Hi all,
I know it took some time...but now I know the reason...problem is solved perfect.

the push rod was too long and did not release the main clininder totally. Please do not ask me why...it was like this simce I have this car.
The push rod is not adjustable...so I turned it shorter on the lathe machine...about 0,7mm ...now it is 0,2mm space in between (was 0,5mm too long)

THANK YOU all for your help here!