1975 FJ40 disk brake upgrade question. Proportioning valve? Yes or no?
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This thread has been a good discussion. Just to add some final thoughts.
I think it's good to experiment a bit and find out if a mod behaves up to spec. I would just advise testing it thoroughly to make sure it doesn't just work under normal driving conditions, but under all conditions.
My personal philosophy is that I don't try to reinvent the wheel, so to speak. Since I'm not an automotive engineer by training, in most cases I would just emulate what Toyota does. You can be assured they have done the homework and the testing, so there's usually a good reason they do things a certain way.
I think it's good to experiment a bit and find out if a mod behaves up to spec. I would just advise testing it thoroughly to make sure it doesn't just work under normal driving conditions, but under all conditions.
My personal philosophy is that I don't try to reinvent the wheel, so to speak. Since I'm not an automotive engineer by training, in most cases I would just emulate what Toyota does. You can be assured they have done the homework and the testing, so there's usually a good reason they do things a certain way.
My "scientific" approach to this: Install the discs (and pull the residual valve in the front brake circuit). Then, do a couple of low speed stops to make sure nothing is leaking under pressure, it's stopping straight, etc. Then, take it up to about 35-40mph and slam on the brakes. If the back end tries to come around on you (the truck tries to spin, or slide sideways) then you need to add a proportioning valve to reduce the power of the rear brakes. If it stops straight, it's probably overkill. Even better is having someone stand a safe distance away, on a gravel surface. Get it up to speed, and hit the brakes as you drive past them. Have them look at the back tires, and see if they're locking up earlier/at the same time as the fronts.
Both sides of the argument, from my perspective:
The original drum/drum set-up, as far as I can tell, has the same residual valves in the front and rear circuits in the brake master cylinder, the same size drums, front and rear, and similar volume wheel cylinders. This leads me to believe that up until 1975, when they did away with drum brakes, the trucks all had a 50/50 split front-to-rear braking system...When my truck still had drums all around, I didn't have an issue with the back end getting squirelly under hard braking - Because of that, I feel like you could probably get away without a rear proportioning valve...
Then again, as others have said, Toyota engineers decided to add one later, so it's probably a safer, more modern setup to add one. An adjustable valve allows you to dial in your system perfectly, for the most safe/effective braking.
Wilwood makes a good, in line, adjustable proportioning valve. I converted to 4-wheel discs, and used one - it seems to do a good job. I think it was about $40.
I built a bracket to strengthen the junction.
Both sides of the argument, from my perspective:
The original drum/drum set-up, as far as I can tell, has the same residual valves in the front and rear circuits in the brake master cylinder, the same size drums, front and rear, and similar volume wheel cylinders. This leads me to believe that up until 1975, when they did away with drum brakes, the trucks all had a 50/50 split front-to-rear braking system...When my truck still had drums all around, I didn't have an issue with the back end getting squirelly under hard braking - Because of that, I feel like you could probably get away without a rear proportioning valve...
Then again, as others have said, Toyota engineers decided to add one later, so it's probably a safer, more modern setup to add one. An adjustable valve allows you to dial in your system perfectly, for the most safe/effective braking.
Wilwood makes a good, in line, adjustable proportioning valve. I converted to 4-wheel discs, and used one - it seems to do a good job. I think it was about $40.
I built a bracket to strengthen the junction.
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He has a '75, but after he mods it, his brakes set up will be similar to a '76 and later truck. For an apples to apples comparison, he needs to consider what Toyota did when they introduced disc brakes in '76.
Have a July 74 Fj40, converted to discs with 10/76 disc components. Used 74 master with removed residual valve. Rear brakes always locked... Quite scary until I added a stock Fj40 proportioning valve.
When I swapped in an Fj45 rear axle, had to switch to an adjustable one to restore balance. And had to readjust it further when I converted rear to discs.
Add the valve, you'll need it IMHO.
When I swapped in an Fj45 rear axle, had to switch to an adjustable one to restore balance. And had to readjust it further when I converted rear to discs.
Add the valve, you'll need it IMHO.
yotamotive
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Thanks everyone for your input. Still not exactly sure what to do. As I said I have the 1986 FJ60 master and prop valve. Then again that truck only had one rear cylinder per wheel as opposed to the two that my 75 FJ40 has. As RWBer said above...some decent testing will be required regardless. I'm leaning towards getting a new master intended for a 76 FJ40 as that is what I will essentially create with my disc conversion. Does anyone know if a 76 FJ40 came with a factory Prop valve?
I tend to lean toward the look and feel of doing things factory. I realize Toyota was always upgrading and doing better as they went along but feel in general that their engineering skills were pretty spot on. I did notice in this conversion that in 76 (where my disc conversion components came from) they hadn't changed the front hub/rotor design yet like the later versions where the lugs went both through the hub and the rotor. In 76 they used six bolts to bolt the rotor to the hub and the lugs are only attached to the hub. Oh well...I trust it...no 35s and V8 on this 40....just an OEM look and feel. Anyway...got off topic a bit there. Thanks again!
I tend to lean toward the look and feel of doing things factory. I realize Toyota was always upgrading and doing better as they went along but feel in general that their engineering skills were pretty spot on. I did notice in this conversion that in 76 (where my disc conversion components came from) they hadn't changed the front hub/rotor design yet like the later versions where the lugs went both through the hub and the rotor. In 76 they used six bolts to bolt the rotor to the hub and the lugs are only attached to the hub. Oh well...I trust it...no 35s and V8 on this 40....just an OEM look and feel. Anyway...got off topic a bit there. Thanks again!
The picture in post 13 shows the stock prop valve from a 76-79 fj40. I am currently doing the same swap, but will be using my 75 master with a prop valve sold by racer and removing my front circuit residual valve. From what I understand the Toyota prop valve is Unobtanium. Racer sells one that is close to stock. It has the metric fittings so you won't need to adapt or change your nuts on your brake line.
Here is the valve that racer sells. It bolts to the stock bracket or a hillbilly one if you like.
If you want a stock bracket call classic cruisers. They sent me one for 15 bucks plus shipping. They probably have a stock prop valve if you want to try a used one.
yotamotive
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Thanks Broth9640. I ended up just using a 1994 FJ80 master with stock boster and no valve. Seems to work great so far! So those two pieces of rubber in the pic...I need those too! Racer sells them?
Yep, PM Racer65 here on mud. Check out the thread Unobtanium to see what else is available.
