FJ40 4 wheel disc (Willwood Proportioning) Too Much Rear Brake!

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***I have searched a lot of threads, I have seen a lot of different resolutions; however, everyone has a different combination of parts. I know there are a million ways to do this, but I am seeking everyones input.***

I have a 1968 FJ40:
- '76 disc front axle with 88 4 runner calipers ( they have the 4x 40mm pistons) I believe.
- On the rear I have my stock '68 axle with the monte carlo Disc conversion kit.
- Single diaphragm mini truck style booster and an 80-Series Landcruiser Master Cylinder
-Willwood adjustable Proportioning Valve


I have the common problem that my back brakes are getting too much pressure and locking up the rear brakes... I have adjusted the proportioning valve in every combination (to ensure I was in fact doing it correctly). I have also confirmed that my brake lines are plumbed correctly.

Should i just add an additional proportioning valve in the rear? Like This?



Any help would be appreciated, I was under the impression that this configuration would work. All parts were new or rebuilt (out of the box) when installed. It is pretty close, it is really only most noticeable on Low traction surfaces (wet, sandy, gravel covered pavement, or gravel or snow)
 

Downey

 
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I'll go out on a limb and suggest you should only be using a proportioning valve like the Summit unit you show above. I have never used anything like the Willwood you show, don't know if it is right or wrong for your application??? The only time I use something like your Willwood photo was when it was non-adjustable, and designed to work with a given master cylinder???
 

pb4ugo

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I used the stk 76 fj40 master with the residual valve removed and stk disc frt end, and of course monte calipers. The rears would lock up as you described. I added the summit valve to the rear circuit and cured the issue.
 
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Before you change again, be certain you front brakes are working properly. I've been down this road on my cars and fixed it often enough that people bring me their problem children to fix (not my day job). I honestly don't know how many times I've fixed this issue. If I were to rank, most of the time there's still air in the line somewhere so the other brakes work, just not very well. Next comes glazing. People keep working and working with their non-working brakes then glaze the ones that were working. Odd, but I've seen it more then once, different pads. Cheap pads tend to last less time, so when someone is mixing and matching one set of pads is really grippy and the other end are a harder, longer-lasting composition (or worse, one is organic the other metallic) so the brake issue is really a pad issue. I've also fixed this issue solely with pads - I had a full size square body (73-87) chevy truck with disk rears that would lock up if you breathed on them wrong - cheap pads in front, metallics in back and it became a joy to drive (ask the BMW guy with no hood how well it stops)...

The Wilwood proportioning valve is far better then the summit one but they both work fine and are dead-nuts simple devices.
 
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Thanks guys, I'm gonna start simple and put new pads around, and bleed the system again, and see where that gets me.... I didn't get to it yesterday, it appears my gas tank is damp on the bottom again, so I tried to look into that yesterday instead! You know, always something to do.
 

lc69hunter

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I have a dumb question. If you have front and rear disc brakes, and a master cylinder from a vehicle with front and rear disc brakes, why would you need a proportioning valve? That is the function of the master cylinder. The only time a proportioning valve should be needed is you are still running drums on the rear.

I did a front and rear disc brake conversion on my 69 (all new brake lines), with a booster off of a 74, and a master cylinder off an fj80, and have had no problems, except for having to shorten the output shaft of the booster, as that was causing the rear to be partially engaged from the get go.
 
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I have a dumb question. If you have front and rear disc brakes, and a master cylinder from a vehicle with front and rear disc brakes, why would you need a proportioning valve? That is the function of the master cylinder. The only time a proportioning valve should be needed is you are still running drums on the rear.

I did a front and rear disc brake conversion on my 69 (all new brake lines), with a booster off of a 74, and a master cylinder off an fj80, and have had no problems, except for having to shorten the output shaft of the booster, as that was causing the rear to be partially engaged from the get go.
Good question - often you don't. A proportioning valve is used when it's a 50/50 split. HOWEVER, and I'll say it again HOWEVER, say the manufacturer determined the car needed a 40/60 (due to the weight balance of the car) split or worse, that the rear drums needed to engage first to steer straight and you blindly put on rear disks without verifying that what's needed is what you're getting.... you'll first experience great stopping but soon your rear pads will cook because that isn't what you need now and you're relying solely on the rear brakes (that are only barely working now).

And to something else, if your car is drum/drum you may already have a master cylinder that would work but you may need to swap the lines so that the front engage slightly first then the rears.

tl;dr - you may not, but you need to design your brake system then determine whether or not the current system, or parts of it, will work for your new system.
 

JohnnyC

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This is the style I got from @Downey about a billion years ago (well maybe 20)

I know that mine I have need to adjust here and there depending on the pads as well as rotors replaced

I don't know about yours but just an FYI these do not go full closed ... it drops to about 50% afaik

IMG_1894.JPG
 
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Another monkey wrench in the works is the braking force applied on each axle. If the mini truck calipers don't apply as much force as the Monte Carlo calipers you'll need a proportioning valve to lessen the pressure to the rear. Since you already have one in place it just may not be enough. Another thing to double check is that you master is the correct non-ABS version and not an ABS version which could cause issues.
 
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You're right, to add to that. It's all about surface area in caliper piston - the larger the piston, the greater potential the force PROVIDED there is adequate fluid flow... Wilwood and SSBC both have awesome web pages that have every calculator you could want to properly size and build your brake system. And conversely, if you don't have enough flow the smaller piston will provide greater force per pedal stroke.... there are books on this stuff.
 
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I have a dumb question. If you have front and rear disc brakes, and a master cylinder from a vehicle with front and rear disc brakes, why would you need a proportioning valve? That is the function of the master cylinder. The only time a proportioning valve should be needed is you are still running drums on the rear.
In theory it could work without a proportioning valve... if the piston sizes were in the correct ratios to match the needed brake bias. I've read of it being done in Europe with Subaru rear discs. However, grabbing montecarlo parts and Toyota parts doesn't achieve this. An adjustable valve is a much easier way to solve the problem. Especially since removing the hardtop can change the balance. I find in a lighter short wheelbase rig, fine tuning is especially beneficial to getting maximum performance out of the brakes. When working right, 4 wheel disc brakes really work (insanely well).
 

Downey

 
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SuperBuickGuy, don't doubt it, but in practice the FJ80 master cylinder with the larger 1 1/16" piston have produced more fluid flow (evidently regardless of pressured) and have proven the sole remedy on many a rig for many a year.
 
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I had no complaints about the 40 master I used for about 15 years with 4 wheel discs. That said the JDM 80 series booster & master I'm running now gives way more assist. I'd highly recommend them to anyone who wants more brakes... or even just a newer master.
 
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Another reason why the master cylinder is a perfectly balanced on the 80 series is that it likely has a balancing valve in the circuit somewhere just like the 70 series trucks do. It adjusts the bias to the front brakes as weight is transferred to the front of the car. It's a great system because adjusts the bias according to how much traction you can get at the wheels as you brake.
 
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SuperBuickGuy, don't doubt it, but in practice the FJ80 master cylinder with the larger 1 1/16" piston have produced more fluid flow (evidently regardless of pressured) and have proven the sole remedy on many a rig for many a year.
I'm not really commenting about specific things that work, just giving tools to figure it out when something doesn't work.
 
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I just picked up a 1975 FJ40 and it is pretty stock but I want to convert at least the front brakes to disc. Is there any sort of all in one kit that I can purchase to do this or will I have to piece together something? If so what should I use? Thanks for your suggestions.
 
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