78 Cj5 Rear Brake Issue

I had an accident on I-75 bridge crossing into Kentucky this spring, wet roads, stop and go traffic, then everyone is doing 55, then stopped dead. The front wheels locked and I slid 4 car lengths, but I needed 4 more ft. Which brings me to my problem. The rear brakes have never been right since the day I bought it. I ordered this Jeep in Sept 77 in Youngstown Oh and picked it up Dec 23rd. And promptly drove to California where we lived until 1990. Several times, on loose gravel or sand I tried to get the rear tires to at least show signs that they were trying to stop, but they always seemed lacking. So at some point I tore into the rear brakes. I routed the rear brake line to a manual valve, by-passed the proportioning valve, straight to the rear. Adjusted the shoes to a slight drag. But to no avail, they seem worse. So since then I have felt I only have front brakes.

The master cylinder has been replaced at least twice in the last 5 yrs. And in 99 during my last restoration I replaced both wheel cylinders.

Any Ideas?

Jay

 

If you read the brake chapter of the TSM, there is a section on the proportioning valve supplied with the CJ-5. This valve's purpose is to hold off the rear brakes in severe braking. Weight transfers to the front wheels when braking, and the harder the braking, the more transfer. Ideally front and rear should lock up at the same time, but the rears locking first is bad. That will put you into a spin if you are turning and braking. If the fronts lock, you lose steering control but you don't spin - instead you go straight ahead until you stop or hit something.

I don't have any quick fix for you. There's something wrong with the rears if you can't lock them up with the combination valve bypassed, like wrong parts or? Maybe the cylinders are undersized? Putting a proportioning valve on the front is not impossible, but there is already a metering valve in the factory combination valve that delays the front application until the rear shoes contact the drums.

 

As I said, this has been a long term problem. And it is possible the factory proportioning valve has been bad from the beginning. I purchased a full set of 1978 service manuals a couple of years after i bought the Jeep. Which is why I tried the manual by-pass I expected the rears to lock up, when they didn't I just left everything as it was as learned not to tailgate I have replaced the wheel cylinders, but have never paid any attention to size or diameter ........ I've just gone to parts store and answered "year make and model" questions from the super highly intelligent, long haired, id--- ...... oops sorry got carried away.

So what diameter should the piston be in the wheel cylinder? Is there an over size, and how would I get it? Or would over size be bad in this case?

Jay

 

I went down and took a pic, and noticed the rear shoe is cracked at the center line, and the front shoe is cracked on the top corner, not debilitating but needs to be addressed.


I routed the feed from the small resovoiur to a manual proportioning valve then down to a Tee one side going to the factory valve, of which the rear is plugged. The other continues to the rear brakes.

I have been thinking about what to do. I have looked into putting disc brakes on the rear, I have also looked at putting electric brakes on the M-416 trailer, I found 10 electric backing plate w/ parking brake capability, Since I'm not good with junk yards, ( most of the local one went out of business during cash for clunkers) Disc's run between $200 - $300 and brakes for the trailer will run $360...... So many options so little money.

Jay

 

Well, looking at RockAuto, it calls for a 7/8" ID cylinder. Looking at the picture, it looks like a conventional wheel cylinder. The brakes look like the same 11"x2" Bendix brakes used on, say, '72-75 CJs. Clearly the backing plates are different, since they fit on the Corporate axle instead of a Dana, but the cylinders could be the same. Rear pistons from a '72 are 15/16", and fronts are 1 1/8" which are 15% larger and 65% larger by area, resp. Rear from a '72 J2500 is 1", or about 30% larger... assuming they are the same style of cylinder. These cylinders are pretty cheap, so it would not be too painful if you had to buy one and compare it.

 

If your shoes are cracking, what's the diameter of the drum? You are only allowed 60-thou oversize, and more than that might bend the shoe enough to crack the lining. Does wear look even?

 

What size rear brakes do you have, 10" or 11" ?
7/8" wheel cylinders are used with the 11"x 2" brakes
15/16" wheel cylinders are used with the 10"x 1-3/4" brakes

I doubt that's your problem but it's what the factory parts book calls for.

 

Yes Drums are 11" and using my crude tools I get 11.015, and the drums and shoes were new 16 years ago. i.e. very little wear, and yes I had adjusted until drag occurred, several times over the years.

But I like the idea of a larger wheel cylinder, ..... "Let's to that" ..... (Galaxy Quest Ref)

Jay

p.s. Thanks Timgr, It's so nice having the wealth of knowledge available here

 

The new wheel cylinders arrived yesterday, and I was look at things and I am worried there may not be enough room to fit the larger cylinder. the 7/8 cylinder is very close to the axle flange.

I'd hoped to get started on this but an abscess tooth has me incapacitated. I should have had it pulled in March, but if it don't hurt leave it alone....right?... OK that was a mistake. Started antibiotics yesterday and called the dentist to get an appt.

Jay

 

Sorry to double post,
I think I will also check the 7lb back pressure valve in the master cylinder, while I have everything tore up, just to make sure it's correct. but I have had several master cylinders over the last 16 years and the rear brakes have been the same.

on a side note, after I put the new shoes on the parking brake needed adjusted, I had my daughter come down and set the brake while I watched the cables, and I found the pass cable moved about 1/2" and the driver cable moved over 2". What I found is the cross bar that pushes the shoes out on the driver side has about 3/8ths or more room before it contacts the shoe, pass side has less than 1/8th.
When I put the larger wheel cylinders on I am going to check dimensions. Are the cross bars different lengths..... are the shoes, from where the cross bar contacts different thickness? "Inquiring minds want to know"

As a temp fix I filled the gap with a screw, ebrake holds fine now.

Jay

 

Dang, put off fixing the brakes for a few decades. Whats a few more days for that toof? Maybe replace the center rubber line to rear axle. Rubber lines have been known to delaminate and seal shut like a check valve with no external indications.

 

That's a thought, I am currently running a flex Stainless steel line, but it is 16 years old The rest of the line on the axle are new, I had to replace them last year, I let the master get low and it sucked air, I went to bleed the brakes but the bleeder broke off, and when I tried to replace the wheel Cylinder, the steel lines broke, so much fun!!! touch one little thing and everything falls apart.

What was it someone said.......

Jay

 

Let's kick'em while they are down!!!!

So,...... I asked my son to give me a hand for an hour he takes the tire off for me, drum removes bolts loosens brake line, I move brake shoes away, remove wheel cylinder, put in new one and try to attach brake line......... When you buy STD brake line it comes with a long nut and a short nut...... since at the time I didn't know better, I bent the line with the short nut at the W/C and long nut at the TEE on the axle. So while the fluid leaks all over the ground, I detached the other side, so I can turn it around................................................................... it's attached to the top differential cover bolt which is less than an inch from the gas tank. My son managed to get the bolt out, I unbent then rebent the brake line, luckily it did not kink. put everything back together, including the top differential bolt. Time 1 1/2 hours. Now after I eat something maybe we'll tackle the other side...... the long nut is in the right place on this side.
Dam@$ P.O. ............................ Oh wait it's a one owner Jeep

 

Splain me why the long nut goes on wheel cylinder side? Someday I may be a PO and dont want to be ridiculed for having done it wrong.

 

Because, I am using front wheel cylinders on the rear. on the front they would have flex hoses that seal via a copper washer and the threads go deeper, because of this the flair seat is put in deeper, thus to get a hard line to seal a long nut is used.,

So even if all for wheels use short nuts , (as mine did) if you replace a line and put the long nut on the wheel cylinder side it will work for any application you may find yourself in down the road. ......... Clear a mud right? I hope not.

Jay

 

Ok. I think I did it right because mine sealed. Took a lot of teflon tape and RTV but no leaks. Want to buy a Jeep?

 

I hope you're joking.....

 

Yeah. I joke. Lots of leaks.

 

Well the larger wheel cylinders are in with no leaks I hope. Bled the brakes. but too tired to test drive today, will do that tomorrow.
The Pass side long nut didn't want to seat, had to really crank on it, It's the one I need to watch for leaks.

Time to clean up, get something to eat and sit here and read you guys.

Jay

 

I HAVE BRAKES!!!

Drove into the back field, slammed on the brake and all 4 wheels locked. This is cool, they haven't done this in decades. The 1 1/8 wheel cylinders did the trick. Thanks All.

Now at some point I will need to work on pavement and get just the right balance with the manual proportioning valve, I might even try going back to the factory valve.

Jay