4 wheel disc???

My friend has an 81' CJ-5 that came factory equipped with front discs and rear drums. Many years ago he installed hydro boost from a 1 ton Chev truck, retaining original MC, and it has worked fine. Now we've added discs to the rear Dana 44 and we can't get any peddle pressure. We even swapped in a new corvette MC but still have the peddle going to the floor. Peddle travels down about 75% then stiffer to the floor and we can hear the pump working harder during this latter travel. Basically no difference between the 2 MC's. We're stumped! I've searched "4 wheel discs" here and have read till my eyes hurt, to no avail.
Any input is greatly appreciated.
Alan

 

The rear calipers typically require more volume than the stock rear wheel cylinders and that is most likely why you are running out of pedal travel, not enough fluid flow. Without knowing more about how the rear was converted impossible to know what advice to give.


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Thank you nick. Just like I told em, I hate being brought in on a project midstream! They say they're GM calipers so I took pics as I can't find any brand on them. We bled em with a mighty vac after teflon taping the bleeders so as to not leak vacuum through the threads, then we got just enough peddle to finish pumping and bleeding again.



New corvette MC

 

Those look like aftermarket Cadillac calipers. Are the bleeders at the high point? If not that is an issue that must be fixed first. If not they are installed upside down ( wrong side). You might also check the Pushrod adjustment between the power booster and master cylinder. If not adjusted correctly you will run out of travel before fully operating the master cylinder.


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Yup, that was the first thing I saw, that the bleeders were at the bottom, so we fixed that right away. We were able to just rotate the whole assembly 180* which placed the calipers behind the axle vs. in front, where they were. Pushrod adjustment "between" PB and MC? I'll check on that next.
We seem to have plenty of peddle travel but that last 25% or so makes an audible strain on the pump as if you were trying to turn past steering lock.

 

From your description you may not be pushing enough fluid to operate the calipers or still have air in the system. I


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After some more trial and error, I've found that the pass side rear mechanical portion of the e brake does not apply any pressure to the disc when fully applied via channel locks. Drivers side works fine so now I'm wondering how these calipers actually are designed. Does the e brake make mechanical contact with caliper piston via a cam setup or does it utilize the actual hydraulic pressure? I'm thinking it would be mechanical contact vs. applying pressure to the caliper. Maybe a bad caliper??
Still stumped here.

 

E brake cable should acuate the piston mechanically. The pistons are not self adjusting. The e brake must be used regularly to adjust the piston. Think it adjusts any time there is more than .030" movement.

 

There is a mechanical lead screw inside the pistol that operates the park brake function of the caliper. Like John said and something I forgot to mention, the park brake should be hooked up and functional for the caliper to work properly.


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Then how do the fronts operate without an e brake or are the front calipers a totally different design than rears? I think we have a bad caliper since I can't get any hold from the e brake on pass rear, yet the other side seems to work just fine. :? Didn't have time to mess with it today, get back on it tomorrow night. Thank you for all your input guys. I'll post up more tomorrow night.

 

The front is strictly hydraulic. Just to be clear, the service brake is hydraulic and park brake is mechanical on the rear calipers with integrated park brake.


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You're on the right track.
Yes the park brake mechanism can affect how the hydraulics work on these.


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Do you have a 2lb residual pressure valve installed for the rear?

 

Sorry guys, been real busy and owner of this problem had a surgery that's kept him down.
Update; Corvette cylinder is back on after retrying original. Installed a Wilwood adjustable prop. valve after the first one we tried was defective because they drilled the mounting holes in wrong place. I can't help but think that the original, frame mounted prop. valve should be eliminated from the system since we now have 4 wheel discs, CMC and an adjustable valve. We now have awesome peddle, but a squeak when peddle is applied which seems to be coming from the adj. valve. Perhaps air in the system still? Also have constant pressure to all 4 wheels while brake is off. Residual?? Is this built into the factory prop. valve or is it an "inline" valve? I know about the 10lb. for rear (drum) and 2lb. for front (disc) norm, so should we have a 2lb. for the rear now as well? I guess I need to know where these residuals are in the system.
Thanks for bearing with,
Alan

P.S. It was my understanding that the corvette MC was set up for disc, hence the residual built in. I'm sure I could be wrong on this one.

 

Typically the residual valve in a stock application is located behind where the brake lines exit the MC. On a stock older Jeep MC, you pull the brake line and run a sheetmetal screw into the flare looking piece in the MC and pull out the valve, which is nothing more than a cone shaped washer with a spring behind it. Can't tell you about the Corvette MC. You need the 2lb residual for the rears as well, otherwise you'll have to pump the brakes every time you use them. I used Wilwood residual valves that I plumbed inline to the front and rear. If the Corvette MC was setup for 4 wheel disc's it may have the residuals already in place, unless Chevy put them somewhere else in the system.

 

Another update,
Never again will I enter a project midstream! Much to my disagreeing, original MC was reinstalled at owners request and still netted identical results as CMC! I then removed new rear calipers and found several problems. What is this and where does it belong?


On one side they had this (spring clip?) sitting between the piston and pad and it was not even present on the other side! Also there was not enough clearance between inner pad and rotor, so these issues were resolved but I didn't reinstall this clip. No residual valves anywhere, as I've read that, on discs with MC mounted above calipers, they're not needed since no springs in calipers to collapse them. I also read that the residual valves are only used on calipers when the MC is mounted below calipers (frame mount, street rod, etc.) so that the fluid doesn't gravity feed back to MC and spread calipers in the process. Made sense to me. E brake is still not up to par, but I'm hoping to deal with that issue after the weekend. Final setup is as follows: Stock 81' CJ MC, 1 ton chev. hydro-boost, stock prop. valve removed and Wilwood adj. prop. valve installed on front brake line (still wondering if it should be on the rear line instead).
Many thanks to you guys for your input! :tea:

At least I had the shop from hell to work in!

 

Usually the proportioning valve would be on the rear. It's used to reduce braking so the rear wheels don't lockup before the fronts on hard braking. I want that shop!

 

Thanks Posi!
I was thinking the same as far as placement of prop valve and you still have me pondering the residual valves as well.
The shop! I know, I'd throw rocks at a house if I just had that shop. It belongs to (Winter X Games snowmobile rider) Joe Parsons dad, Bill Parsons. It's 60' X 50' with every tool imaginable, pool table, nice big "bar". The table in center where wrenches hang is 1"x4'x 8' steel attached to the floor of course, with power and air on both sides.

 

Just something to think about, but make sure when your swapping MCs around that front and rear brake lines go to the correct port on the MC. Not all MCs have the front brakes going to the front port. Had a local guy fighting a similar issue and he had his lines swapped.

I allways hated rear calipers with built in e-brakes when I was a mechanic. Make sure they are adjusted properly.

Nice shop!

Kevo